Here are a few theories/speculations/predictions I have come up for the upcoming second movie. You can take it seriously or you can laugh it off:
1) Kenny's siblings Karen and Kevin will play a major role. (Karen will probably help our Stan and Kyle)
2) A very controversial one: Randy getting killed off in the movie for all the crap he has done since Season 22.
3) The past Cartman freaking out at what he has become in the future and help our heroes out.
4) The future Stan tell his past self to stand up to Randy. Bonus if Stan delivers a "Reason You Suck Speech" at his dad.
5) As I mentioned earlier, it's time Kenny's brother Kevin have a proper role in the show rather than being a "living prop".
6) Expect a few references of time travelling movies like Back to the Future, Avengers Endgame, Tenet, The Terminator and Looper.