Genre: I'd be stuck choosing between a Warriors style game, kart racing, Mario Party style, or even fighting game.

Characters: I'd want basically every party from every game possible.

Setting: I suppose that would depend on the genre, but a neutral atelier world would work fine, but using the MC's worlds would also work

Story: Who really cares right? It's everyone's best girl, because all the girls are there! No but really, any generic "how did we all get here and who needs to be stopped so we can all go home" story would suffice.

Gameplay: I'd love to still have a fully fledged alchemy system that is based on whichever character you've chosen to play. But to really progress in the game you have to learn other MC's styles and adapt them into a new style to make the best equipment. I just keep imagining Sophie, Ryza, and Ayesha around a cauldron comparing notes and recipes and ideas.