I hope that one can develop romantic relations with their partner. Also would like if some missions required more than the player and their partner. Maybe multiple teams of two working together? That should open up a lot of room for character development. For extra complexity points, you can make some of the other teams from characters the player didn’t choose as their partner. Who they are and how well they cooperate could depend on who was available for them to partner with. Could lower the complexity level by making their teams non-permanent, meaning John can partner with Sue during one mission and Ryan during another. If a second team is supposed to tag along at some point, you can program in two “normal” members, and a third “replacement” character that replaces one of the two normal members if they are chosen as the player’s partner.
Not sure how you plan on controlling their powers. The easiest approach is the generic “all hunters are like this, all guardians are like that.” Bonus replayability if you go for “hunters are either A, B, or C, and guardians are D, E, or F”. Super ultra omega bonus replayability and interest if you go for the (difficult to implement with lots of potential partners) “holy cow there are so many random powers - he can manipulate time and she can walk through shadows and that other guy can turn his hands into giant bees that shoot superheated lasers” approach.
Anyway, I look forward to the demo. This sounds like fun.