"Summer line-up" means...

If nothing else this introduced me to Ava Max and her music. It's quite delicious. I'll take a pack.

My dream would be a Themepark EP as one of the Summer Lineups they announce.

They'll officially announce the leaked kit that will be released the day after.
Moreover, they will give an outline of how many and what kinds of packs we can expect during the next 6 months (e. g. 1 GP, 1 EP and 1 SP). I doubt they'll tell us about the themes.
I also think they will not tell us about upcoming kits and/or patches in the summer-line-up. They will want to keep their flexibility with them both.

I guess the SP will be the the grim reaper based stuff pack that has been voted on a while ago. I saw on of the new options in Lilsimsie's Chaos Let's Play a while ago. My uneducated guess is that we will get the SP in beetweed the GP (probably June) and the EP (probably October/November). I think we might also get a bigger gameplay patch in between the GP and the EP.

I bet it's gonna be the Big summer expansion pack where you have cars and campers and burglars looting your home whilst you are on vacation. Then a game pack with racing as it as it's theme. A helicopter kit and a lot kits like the courtyard kit so you can build holiday destinations. It's gonna be so awesome

I'd guess at least one of each pack type + maybe some info about patch features, if there's something major that's not coming in May patches (such as another iteration of bunk beds or the leaked thing with sofas working like counters).

https://yt3.ggpht.com/QAADomFTydxKSaLLGZ3YHa8ooap4w3OIOpAgF997lzRwhTvlDE0c7HCQFFINUdU_zKLtD517oy_0aw=s1600-nd

https://twitter.com/LosSimsES/status/1392889750059634691/photo/1

SimsES confirmed the kit

"ChocoSulSul;c-17879471" wrote:
https://yt3.ggpht.com/QAADomFTydxKSaLLGZ3YHa8ooap4w3OIOpAgF997lzRwhTvlDE0c7HCQFFINUdU_zKLtD517oy_0aw=s1600-nd

https://twitter.com/LosSimsES/status/1392889750059634691/photo/1

SimsES confirmed the kit

Onda vital a todo gas lobezno

It was already confirmed on Instagram as well

"Mak27;c-17879480" wrote:

"ChocoSulSul;c-17879471" wrote:
https://yt3.ggpht.com/QAADomFTydxKSaLLGZ3YHa8ooap4w3OIOpAgF997lzRwhTvlDE0c7HCQFFINUdU_zKLtD517oy_0aw=s1600-nd

https://twitter.com/LosSimsES/status/1392889750059634691/photo/1

SimsES confirmed the kit

Onda vital a todo gas lobezno

It was already confirmed on Instagram as well

Hahaha! no no. Here it is Kame-hame-ha! Rapidos y Furiosos and Logan. I am from Argentina.

Considering this. We could think that the GP will be announced on the 17th and maybe the EP?

"ChocoSulSul;c-17879585" wrote:

"Mak27;c-17879480" wrote:

"ChocoSulSul;c-17879471" wrote:
https://yt3.ggpht.com/QAADomFTydxKSaLLGZ3YHa8ooap4w3OIOpAgF997lzRwhTvlDE0c7HCQFFINUdU_zKLtD517oy_0aw=s1600-nd

https://twitter.com/LosSimsES/status/1392889750059634691/photo/1

SimsES confirmed the kit

Onda vital a todo gas lobezno

It was already confirmed on Instagram as well

Hahaha! no no. Here it is Kame-hame-ha! Rapidos y Furiosos and Logan. I am from Argentina.

Considering this. We could think that the GP will be announced on the 17th and maybe the EP?

Lol soy de Chile, cebate unos mates!!

Todo es posible, habrá mas de un anuncio // Everything it's possible, it will be more than one announcement

I voted for GP or Kit reveal but honestly it could also be a SP or EP or base game updates or a combination of all these things. I usually stay away from the community as far as speculation is concerned because I've learned to lower my expectations as well as not listen to hype or drama when it comes to potential future content for the game. It saves me the headache.