I feel level 12+ just gets bonkers in D&D really fast (especially when you start adding in even more powerful magical items). You start to get to demigod\legendary levels where your party would/could alter the lore of the entire setting. I feel 12 is a great stopping point for BG3 and even if introducing more DLC, keeping the max level to 12 would be fine. Unless the DLC has you go off to the various layers of Hell or another "off world" type setting where things would dramatically scale up.
I'd love to see DLC for BG3 to add in more races, classes, companions, and minor storylines (like a procedurally generated dungeon you could run at any time and it could be different each time, to add to replayablity). Hopefully, Larian is making big $ with BG3 to fund both teams for long-term BG3 support and DLCs and also fund their own/next projects.
