Well it is up to Larian decide what they do. However regardless how it has been with DOS2.
Simple fact remains BG3 will not be even near level 20 for full game that is available. I have said many time I do not care for high end gaming so much in DnD and would be fine with example max level 12. What is max level in BG3 during full release? I do not know, but my guess between level 11-14.
With an expansion big one to BG3 they could increase max level even to perhaps level 20. Now if they would do such thing I do no think BG3 would get get that any time soon not during year 2023.
Finally it is up to Larian what they do. I can not promise what Larian will do or not do they have the power to decide. That said from Dungeons Dragons view to bring maximum level to 20 would kind of make sense. Now and I do not want to hear of Pathfinder Epic levels beyond 20 thank you. It is very possible BG3 will not get to even near level 20 never. Compare at Solasta is it maximum level 20? No maximum level in Solasta expansion pack Lost valley is 12. That said in Lost Valley Solasta DLC to buy you begin from level 1 again so it is a journey from level 1-12 so Lost valley does not feel very short.
Finally from BG3 worst view what could happen is that Larian decide they will develop something completely different after BG3 that has nothing to do with BG3 and no more content.
As for modding or GM possible you know I do not believe in GM in BG3 before I see it sounds a bit like a pipe dream to me seriously. I know Neverwinter Nights had that, but currently BG3 mods what can be done is very much behind what Neverwinter Nights 1 and 2 you could do it is very big difference indeed. You could even create Online private servers worlds with much more players then 4 players multiplayer in Neverwinter Nigths 1 and 2 and that is even more advanced then a GM mode possible, but yes it included in GM as well in NWN 1 and 2.
