One thing I'm really hoping for and that I'm sure a lot of people have been saying/thinking as well, is for Byleth to be a good character.
It sounds pretty obvious, considering the fact that no one would want a bland and boring character, but Byleth has so much potential and I really don't want it to go to waste. Being a teacher and leading your students into war wouldn't be easy at all, seeing as you'd have the lives of all of them weighing on your shoulders. You'd have to be stressed out of your mind eventually, worrying about losing the people that you taught and made bonds with.
It's sort of hard to explain, but I think the idea is clear enough. Overall, I just really don't want Byleth to be all "meh" about sending the students into battle. I think seeing some concern or worry from him, especially if it's before a major battle in the story or something like that, would help characterize him a bit more.