I don't think there will be another expansion, and there isn't going to be a way to import save-games into Divinity 3. Even in Dragon Knight Saga, the way you completed certain quests (Folo's pigs, Willy's house) had no impact in Dragon Knight Saga.
And spoilers for the ending:
It makes no difference if you free him or not. If you don't, he stays locked up, If you do, you fight him as well as the final boss together and he dies (again).