Let's see...
1. The return of support conversations.
2. Branching class paths, like in FE8.
3. Weapon forging (this seems like a given, but I'm not totally sure.)
4. New continent and story.
5. Multi-tier promotions, like in FE10.
6. Being on the Wii (or its successor)
7. More cutscenes/voice acting.
8. If skills are included, less broken mastery skills. Radiant Dawn and Path of Radiance were way too easy with them.
9. Trial maps.
10. Free DLC trial maps, and maybe characters for them?
11. As other people have said, a lord that doesn't use swords. I'm not particularly demanding with this one, but it would be nice.
EDIT: Thought of a few more.
EDIT 2: I have since discovered how to properly use reclass (I know pretty much nothing about growths and whatnot, I just play the game), so I decided to remove my comment about that.