I would also assume Moonrise Towers is the end of Act I. They seem to follow the traditional 3 act story structure so the big turn of events after first Act would be learning what's really going on, and Moonrise would be the place for that.
We have seen a cinematic where Baldur's Gate was attacked, and we have also seen a dream sequence about the city burning. So the grand finale of the game could actually be a massive invasion on Baldur's Gate. Saving a city state of that size would be level appropriate finale for BG3 end levels (11-12?).
Another theory could be that Act III is about the rise of the dead gods after their Mind Flayer minions have been dealt with at the end of Act II and saving the city could come earlier. So the stakes would get higher for the final act.
I just hope Larian is not going to mess this up and have us fighting for the city before we get to know it and get attached to places and people. Ideally both Acts II and III could use Baldur's Gate as a hub. We would get to explore every nook and cranny of the city, have some down time there, meet people we like, and visit other locations like the Nine Hells and the Astral Plane from the city.
