I don't think there is a principal issue with extending the level cap, but at the same time I do believe more than level 12 is too much for a single campaign.
BG 1 took you to about level 7, BG 2 took you to about level 15-20 depending on class, and only ToB took you to 30.
For even older players, it took four games to reach those levels (looking at the Pool of Radiance to Pool of Darkness sequence.
The issue with doing it in a single campaign and so fast is that you basically upgrade gear all the time, which sounds fun but makes special loot less special.
Level 12 is fine. Level 14 would be fine with a medium-sized DLC. Level 20 would be good if there was a sequel you could import your character to, but that is not planned.
As for this part:
There's been talk about how levels 13 to 20 are too powerful for a game to really explore. I find that hard to believe.
Such has been done in previous games, for one.
But perhaps more importantly, we are *way* more powerful at level 12 than characters would typically be in the higher levels. The tadpole powers and the equipment... it's all off the charts. Blackhole, stage fright, displacer beast, favourable beginnings, I mean, the list goes on and on. My character is nigh unstoppable after consuming all the tadpoles.
This makes the higher level worse, not better. Those extra illithid power act as a multiplier on your level. If a normal party of level 16 characters is hard to balance, then giving them extra powers on top only makes things even worse. As does the homebrew of Larian - a Sorcerer casting a level 9, 8 and 7 spell in a single round is frankly insane.
