I'm replaying PoE with both expansions (first time with the expansions - criminal really since I've had the game on release and finished it twice before the expansions hit) and really enjoying it but one thing I would love to see expanded upon in Deadfire is the concept of "wizard duels".
It's a fairly small thing to implement but I'm thinking of Baldur's Gate 2 specifically when I think of this - there were a few times throughout that game where magic users talents were brought right to the fore. A few times in climactic battles but a few where your sorcerer/wizard basically has to go one on one with another magic user. The most blatant example of this was in the Underdark where you could take on a series of escalating duels for the entertainment of some tavern patrons.
But only your wizard was allowed to fight, and the opponent was also a wizard.
This would be awesome in PoE as there are a lot of spells designed to counter/absorb spells and effects but in the middle of the hectic combat of PoE you rarely get a chance to see them used effectively.
I think weaving something like this into a questline would be pretty awesome.
Again just my suggestion but please feel free to comment!