I went thinking about the most important and memorable stories for me in this game, and realized that most of the lines that mattered most to me have been covered. Two types, though, haven't been touched so far as I am aware.
Relic weapons-- they are NPCs, and for much of the community, they were either a dream out of reach, or (back in the day) the pinnacle of success, our greatest moment of achievement. And while the question of WHICH relic is open, all of them lead very easily to two sphere functions. My nod is to Mandau-- crit hit rate, crit hit damage. Best, this thinking could easily lead to an entire line.
Crafting is the other thing near-and-dear to my heart, and I'd love a trust to come out and help with that. Mewk Chorosap for HQ Rate and Materials loss down (since kitron macarons pretty much cap success rate), would be fantastic, as the only character that I'm aware of that interacts with all crafts right now.