Well assuming you're planning to limit your world to only vanilla EP/SP content (with minor mod use using support of Twallan's mod suite and utility mods like Jynx RH) etc. How about:
1. A remake of Venice World.
Restrictions: Must use 100% of the content from the Monte Visa world and Last Venue of Amore set. Any supporting marine based content from the IP EP permitted. And 100% of the road network must be canal based to use the Amore and/or IP marine content. All other EP/SP content must be Italian themed. A world similar to Monte Vista in scope but more authentic in layout like VENEZIA
http://forums.thesims.com/en_US/discussion/919078/v-e-n-e-z-i-a-realistic-venice-world-wip
http://forums.thesims.com/en_US/discussion/919078/v-e-n-e-z-i-a-realistic-venice-world-wip/p1
Venice Lagoon world with distinct neighborhoods (best Venice to date IMO)
http://modthesims.info/d/396210/venice-lagoon.html
and post card perfect chic of Positano
http://www.thesims3.com/assetDetail.html?assetId=4283633
...while staying true to the authenticity and scope of historical Venice in Assassin Creed 2: Brotherhood (yes tall order but you said you wanted a challenge!)
2. SimAtlantis.
Restrictions: challenge is 80%-90% world map is underwater. Must use 100% of IP EP content. Can use whatever WA EP content necessary. The majority of the sim population are mermaids by heredity default. Those unfortunate sims lacking the mermaid trait (your playable sims, townies or random service sims the game generates) will be forced to live/occupy the remaining 10-20% of the existing land mass. So discretion on what EP/SP content needs to be considered due to size limitations. Weather/Climate restricted to Summer using Twallan's Tempest. With allowances for a freak Winter blizzard or Spring monsoon season over a brief 1-2 sim week, just to keep things interesting in the neighborhood.....
3. A remote tree house village in Star Wars Ewok style.
Restrictions: All lots would need to be placed on elevated, flat plateau mounds connected only by bridges. Must use LN EP bridges for road network. Mod customization of vanilla road bridge network (i.e. wooden bridges to look like Ewok house tree bridges) is permitted. Map should have less than 30% natural fresh water, has no salt water, is a jungle themed green & eco friendly village. Swimming pools and manually created water holes on lots are permitted. Must be vegetation lush/tree friendly. Tropical rain forest restricted to Spring and Summer weather in Tempest. Limited to exotic animals (lizards, parrots etc) from Pets EP. Limited to ethnic friendly content from WA EP, and store sets like Safari living, King of Cambodia, and any other supporting jungle/safari themed content (i.e. content from worlds like Barnacle Bay is a no go as it's not jungle/Amazon forest theme friendly).
4. Proper, functionally useful remake of city of London. No. This isn't a resurrection of all the London Project forum drama or reasons why this amazing looking SimCity replica of the real city failed to function as it should. The challenge lies in whether a fraction of this world with a similar layout (and a significantly less dense building design) could be accomplished to support a default 200 townie sim population. No restriction on any EP/SPs needed in world design
5. An 1850's western frontier farm based, rural town/village.
Restrictions: Pre industrial western frontier in a Northwestern, Southwestern or Prarie country farmland theme. Only foot traffic and/or horses used for transportation. Roads limited to unpaved/dirt roads so no paved/cobbled roads to encourage spawn of modern traffic. Must use farm themed friendly sets like Farm Fresh Folk and Grandpa's Grove sets. All EP/SPs can be used in support to build this. Weather is seasonal in Tempest. But if world is Northwestern Rockies themed then map must be mountainous and subject to a significant winter season. If Pacific coast/Northwestern forest based then rain/Spring should be the more dominant seasons
IF USING MODS
6. A Roaring 20's Simtopia in the Art Nuevo Decor style. Only restriction is use of all content from Roaring Heights world. And with antique 20s-30s cars available from the store and modders like Fresh Prince. Sites like ATS3 Sandy, MTS etc are best for supporting world content.
I've got zero idea if any of these worlds are remotely feasible from a design stand point. But it would be great if they could be made. Since this rambling post makes it fairly obvious I just woke up, and have too much time on my hands posting this on my day off :D
edit: Forgot to add this. Since you're into berry sims, an entire world dedicated to an amusement park the likes of Disney world. Complete with all amusement themed content. (perfect way to use 100% of the content from the Sweet Treats SP IMO). So for example: dedicate each lot to a specific amusement content. Like a lot for the Ferris wheel, the Merry go round, surfing/wave station, petting/riding zoo for kids (using Pets EP), reptile/avian houses, waterworld pools, seasonal festival content like snow cone/pop corn stands, etc etc. With highly customized lots having exotic building designs like your delicious looking Cake worlds <3
Restrictions: must be a flat world to permit global, unhindered use of children equipment like strollers around the map.