countries and maps of Battlefield 1

Bulgaria
The Kingdom of Bulgaria was one of the Central Powers in World War I, and the last to join the alliance.

a fact related to Bulgaria on the loading screen:
The nation that mobilized the greatest percentage of its population in the war was Bulgaria.

another fact about Bulgaria on the loading screen:
Irish troops served not only on the Western Front, but even fought Bulgarians in the Balkans.

3 skins are associated with Bulgaria in Battlefield 1 (Monastir, Cobadin and Crna Bend skins)
-Monastir skin for Mosin-Nagant M91
During World War I Bitola (Monastir) was on the Salonica front. Bulgaria, a Central Power, took the city on 21 November 1915, while the Allied forces recaptured it in 1916. Bitola was divided into French, Russian, Italian and Serbian sections, under the command of French general Maurice Sarrail. Until Bulgaria's surrender in late autumn 1918, Bitola remained a front line city and was bombarded almost daily by air bombardment and artillery fire and was nearly destroyed.

-Cobadin skin for Mosin-Nagant M91
The First Battle of Cobadin was fought from 17 to 19 of September 1916 between the Bulgarian Third Army and the Romanian–Russian Army of the Dobruja. The Second Battle of Cobadin took place from 19 to 25 October 1916 between the Central Powers, chiefly the Bulgarian Third Army, and the Entente, represented by the Russo–Romanian Dobruja Army.

-Crna Bend skin for Vetterli-Vitali M1870/87
The Battle of the Cerna bend (1916) was a two-month-long battle between the Bulgarian and the Entente armies. The battle took place in the Macedonian front during the First World War Allied Monastir Offensive in October and November 1916.

Bulgaria participates in 6 Battlefield 1918's maps:
-Albania, October 1915 (Serbia vs Bulgaria)
-Battle of Bazargic, September 1916 (Serbia vs Bulgaria)
-Battle of Struga, November 1915 (Serbia vs Bulgaria)
-Hill 1050, May 1917 (Italy vs Bulgaria)
-Kosovo, November 1915 (Serbia vs Bulgaria)
-Morava Offensive, October 1915 (Serbia vs Bulgaria)

my ww1 movie recommendation with Bulgaria or the BF1 Crossbow Grenade Launcher's cameo role
Capitaine Conan (1996)
https://www.imfdb.org/wiki/Capitaine_Conan
Capitiane Conan is a 1996 French war drama movie directed by Bertrand Tavernier based on the 1934 novel of the same name by Roger Vercel. In September 1918, for four years the First World War raging in Europe coming to an end. In the Balkans, the Bulgarians allied with the German Empire and the Allies deliver tough battles (Macedonian Front). In the last battle before the surrender of Bulgaria the Sokol massif is stormed by the Allies and Captain Conan and his fifty comrades, mostly ex-prisoners from military prisons, have played a decisive role. This is a special unit in the regular army hierarchy, which operates according to its own ruthless rules.

Crossbow Grenade Launcher (Captain Conan)
https://www.youtube.com/watch?v=6xd770k7u6g

Kingdom of Romania
Romania allied with the Entente Powers during World War I.

there is a reference to Romania in the Fedorov Avtomat's Codex Entry:
The Russian Army adopted the Fedorov Avtomat in 1916, and it was first used in combat on the Romanian front, for example by a special unit of the 189th Izmail infantry regiment which were armed with 45 of the 6.5mm Fedorov Avtomats, and 8 of the earlier 7.62mm self-loading rifles.

3 skins are associated with Romania in Battlefield 1 (Marasti, Oituz and Hohenzollern skins)
-Marasti skin for Fedorov Avtomat
The Battle of Mărăști (Romanian: Bătălia de la Mărăști) was one of the main battles to take place on Romanian soil in World War I. It was fought between 22 July and 1 August 1917, and was an offensive operation of the Romanian and Russian armies intended to encircle and destroy the German 9th Army. According to General Alexandru Averescu, the commander of the Romanian Second Army at this major engagement, the Battle of Mărăști was the "first true victory in the history of the modern Romanian Army".

-Oituz skin for Fedorov Avtomat
The Battle of Oituz may refer to any of the following battles between Austro-Hungarian and German forces against Romanian ones during World War I:
First Battle of Oituz, 12–27 October 1916
Second Battle of Oituz, 10–16 November 1916
Third Battle of Oituz, 8–22 August 1917

-Hohenzollern skin for lMG 08/18
https://en.wikipedia.org/wiki/Ferdinand_I_of_Romania
Ferdinand I (Ferdinand Viktor Albert Meinrad; 24 August 1865 – 20 July 1927), nicknamed Întregitorul ("the Unifier"), was King of Romania from 1914 until his death in 1927. His family was part of the Catholic branch of the Prussian royal family Hohenzollern. Ferdinand became King of Romania on 10 October 1914, under the name Ferdinand I, following the death of his uncle, King Carol I. He ruled Romania during World War I, choosing to side with the Triple Entente against the Central Powers. This led to Kaiser Wilhelm II of Germany removing his name from the royal house of Hohenzollern.

Romania can be found on a Battlefield 1918 map:
-Transylvania, August 1916 (Romania vs Austria-Hungary)
https://en.wikipedia.org/wiki/Battle_of_Transylvania
The Battle of Transylvania was the first major operation of Romania during World War I, beginning on 27 August 1916. It started as an attempt by the Romanian Army to seize Transylvania, and potentially knock Austria-Hungary out of the war. Although initially successful, the offensive was brought to a halt after Bulgaria's attack on Dobruja. Coupled with a successful German and Austro-Hungarian counterattack which started in mid-September, the Romanian Army was eventually forced to retreat back to the Carpathians by mid-October.

a very special Romanian World War I weapon:
Fahrpanzer
https://en.wikipedia.org/wiki/Fahrpanzer#Romanian_infantry_variant
The Fahrpanzer was a mobile artillery piece made prior to World War I in Germany, implemented in several German fortifications from 1890 onwards and exported to several foreign military powers prior to the outbreak of hostilities. Romanian infantry variant: Romania purchased 334 Gruson Fahrpanzers, in the 53 mm caliber. These guns remained in their emplacements for about twenty years, before being transformed into infantry guns between 1914 and 1916 by mounting them on Romanian-built gun carriages. A few were transformed into anti-aircraft guns.

the BF1 loading screen has this historical fact:
Austria-Hungary issued recruitment posters in 15 different languages

I recommend the following World War I Romanian film to watch:
Forest of the Hanged (novel)
https://en.wikipedia.org/wiki/Forest_of_the_Hanged_(novel)
Forest of the Hanged (Romanian: Pădurea spânzuraților) is a novel by Romanian writer Liviu Rebreanu. Published in 1922, it is partly inspired by the experience of his brother Emil Rebreanu, an officer in the Austro-Hungarian Army hanged for espionage and desertion in 1917, during World War I. The novel was made into a film in 1965. The film was directed by Liviu Ciulei, who won the award for Best Director at the 1965 Cannes Film Festival. Plot: The protagonist is Lieutenant Apostol Bologa, who was born and raised in Parva - then Párva, Beszterce-Naszód County, Transylvania, Kingdom of Hungary. Although he was enrolled in the Philosophy Faculty of the University of Budapest, and he had not been conscripted into the army since he was a widow's son, Bologa volunteers into the Austro-Hungarian Army at the start of World War I. After attending artillery school, he is sent to the front. He fights valiantly in Italy and Galicia; wounded twice in the next two years, he is promoted to the rank of lieutenant and decorated three times. Sent on the Romanian front, in the Eastern Carpathians, the thought of desertion becomes an obsession for him.
https://www.youtube.com/watch?v=7aWbJ-H5e8o

study the following EA infographic
the meaning of the star: New fronts added by expansion packs.

I found a Battlefield 1 fanfiction DLC idea ("The War at Home"), which I think was inspired by "The Young Indiana Jones Chronicles" episodes.

Battlefield 1: The War at Home
https://battlefieldfanfiction.fandom.com/wiki/Battlefield_1:_The_War_at_Home
Battlefield 1: The War at Home is a fan-proposed DLC for Battlefield 1 based on revolutions and sabotage in World War I, including the Mexican Revolution, Easter Rising, and German Revolution. The War at Home DLC brings the fight to surprising and unique corners of the globe, including Mexico and Ireland. Ride across the American border with Pancho Villa, battle rebels on the streets of Dublin, and stop saboteurs within sight of the Statue of Liberty!

Maps:
-Raid on Columbus
-Celaya
-Easter Rising
-Ashbourne
-Black Tom
-Kiel Mutiny

Where is the "The Young Indiana Jones Chronicles" connection?

-"Young Indiana Jones and the Curse of the Jackal" → Spring Break Adventure
Adventures in Columbus: On March 9, 1916, Indiana Jones and his cousin Frank visited the town looking for "girls but they were caught in the middle of Pancho Villa's raid on the town, in which the Sentry was shot. To this day, the Villista attack on Columbus stands as the only attack on US continental soil by a foreign army since the War of 1812. Pancho Villa had ordered the attack the town, after the United States had officially recognized the Carranza government of Mexico.

The Young Indiana Jones Chronicles – "Ireland, April 1916"
The Easter Rebellion or Easter Rising of 1916 was a violent confrontation between British forces and Irish Fenian rebels seeking an independent Irish republic. Although the Fenians were brutally defeated, many dying during the battle and several others executed by firing squads afterwards, the method of suppression helped sway public sympathy toward the rebels.

"The Lion of Africa" fan proposed DLC was inspired by two facts of the BF1 loading screen?
Here is the description of the fictive "The Lion of Africa" DLC:
https://battlefieldfanfiction.fandom.com/wiki/BF1:_The_Lion_of_Africa
The Lion of Africa is a fan proposed Battlefield 1 DLC based on the African front of the First World War, including the Battle of Lake Tanganyika and the exploits of Paul Emil von Lettow-Vorbeck, the German master of guerilla of warfare also known as "The Lion of Africa".

first fact of the BF1 loading screen:
A small group of German soldiers under General Von Lettow-Vorbeck fought a guerilla war in German East Africa against Allied forces. They held out for the entire duration of the war.

second fact:
The British Army transported two boats through the jungle to fight the Germans on Lake Tanganyika.

I think the Battle for Lake Tanganyika was made famous by the movie "The African Queen" (or probably the "Jungle Cruise")
Battle for Lake Tanganyika
https://en.wikipedia.org/wiki/Battle_for_Lake_Tanganyika
The Battle for Lake Tanganyika was a series of naval engagements that took place between elements of the Royal Navy, Force Publique and the Kaiserliche Marine between December 1915 and July 1916, during the First World War. The intention was to secure control of the strategically important Lake Tanganyika, which had been dominated by German naval units since the beginning of the war. The British forces – consisting of two motor boats named HMS Mimi and Toutou – were under the command of the eccentric Lieutenant-Commander Geoffrey Spicer-Simson. The boats were transported to South Africa and from there by railway, by river, and by being dragged through the African jungle, to the lake.

https://www.youtube.com/watch?v=gYBDRAp4Or8

I think the "War in the East" fan-proposed DLC was partly inspired by SMS Emden's story.

https://battlefieldfanfiction.fandom.com/wiki/BF1:_War_in_the_East
War in the East is fan-proposed DLC for Battlefield one based on the various actions in the Far East and Pacific Ocean during the First World War, including the Siege of Tsingtao, the Australian invasion of German New Guinea, and the Battle of Penang.

maps:
-Tsingtao
-Penang
-Mücke's Stand
-Cocos Islands
...

the adventurous story of SMS Emden:
https://en.wikipedia.org/wiki/SMS_Emden
SMS Emden ("His Majesty's Ship Emden") was the second and final member of the Dresden class of light cruisers built for the German Kaiserliche Marine (Imperial Navy). Emden spent the majority of her career overseas in the East Asia Squadron, based in Qingdao (Tsingtao), in the Jiaozhou Bay Leased Territory in China. In 1913, Karl von Müller took command of the ship. At the outbreak of World War I, Emden captured a Russian steamer and converted her into the commerce raider Cormoran. Emden rejoined the East Asia Squadron, then was detached for independent raiding in the Indian Ocean. The cruiser spent nearly two months operating in the region, and captured nearly two dozen ships. On 28 October 1914, Emden launched a surprise attack on Penang; in the resulting Battle of Penang, she sank the Russian cruiser Zhemchug and the French destroyer Mousquet. Müller then took Emden to raid the Cocos Islands, where he landed a contingent of sailors to destroy British facilities. There, Emden was attacked by the Australian cruiser HMAS Sydney on 9 November 1914. The more powerful Australian ship quickly inflicted serious damage and forced Müller to run his ship aground to avoid sinking. Out of a crew of 376, 133 were killed in the battle. Most of the survivors were taken prisoner; the landing party, led by Hellmuth von Mücke, commandeered an old schooner and eventually returned to Germany. Emden's wreck was quickly destroyed by wave action, and was broken up for scrap in the 1950s.

A German WW1 film was made in 2012 based on the story of SMS Emden:
13.000 Kilometer - Die Männer der Emden ("The Men of the Emden", English title: Odyssey of Heroes)
https://www.youtube.com/watch?v=PZ_qTt0h_fY