Larian said they were releasing a complete game. I honestly hope they don't release an expansion. The ending was great, and I don't know where you go after. An expansion past the ending would almost cheapen the whole game for me. After a huge build up to the fight of the netherbrain. It wouldn't feel right to continue after that.
And only getting to 12th level felt fine. I don't think it needed to go higher. Mine abs my partner's characters were beasts even at level 12. So it doesn't feel like it's necessary.