I kinda just wanna see Grima again. His portrayal in Awakening was kind of bland (which bugged me because he had good potential), but the post-game of Echoes built him up way better than Awakening did and really want to see something done with that dark storytelling. If that managed to impact the backstory of Awakening, imagine what a game would do. I was almost entirely expecting FE Switch (now Three Houses) to have done something with it before its announcement, so I'm still holding out for my Grima closure.
Returning from Three Houses, I would like the character development seen with them. The characters in Three Houses had a surprising amount of depth for being tropes, and I do appreciate that. Support conversations are okay in revealing depth and growth and I don't mind learning about the characters in that fashion, but I would also like to see more of what Three Houses did in that the events really affected how these individuals changed as people. A lot of the early missions involved taking on some of the students' familial figures and the perspective change per house showed different sides of their responses, which was excellent. I get waifu sells, but characters are more unrelateable the further into that territory they tread.
I am all for routes, but Three Houses' routes leave something to be desired. I've only finished one all the way through, but based on what I've heard from friends who did the other routes, all the routes just cut off the story and magically resolve everything during the credits roll instead of giving us proper conclusions. I also dislike how much things change just because Byleth is with a different group. One person would not change that much; I would have liked to see the same or similar events played out just from different perspectives. I realize that would mean only one group would get "dibs" on the Monastery as their hub post-timeskip, but that makes the most sense to me. Overall though, I think it was a way better take on the route concept, and I would be okay seeing it again in a less extreme fashion. As long as we get an actual conclusion, please.
I feel the same as most about the freeroaming hub concept, I wouldn't mind it staying. I spent way too much time between missions; fishing in Fire Emblem is something I didn't know would exist... and somehow it still felt Fire Emblem? Idk, man. But anyway, I kinda miss having a proper world map, and some sort of hybridization of that with the hub would be cool.