I started thinking about what I want out of a new FE, and my biggest request is that it be replayable. This is one of the greatest strengths of Fates and Awakening. The class-changing system and the child units mean that a second and third play through can each be unique, while the GBA games were the exact same on each playthrough. With that in mind, I created a small list of things I'd like to see in FE:Switch.
1) Lots of Seals. I preferred Fates' class system to Awakening's solely because MyCastle gave me an unlimited supply of Seals. This meant that I can play through the game multiple times and never have a unit be the same. Awakening also had a nice class system, but the limited access to Seals until mid-game made the game relatively monotonous until then.
2) Multiple armies. I absolutely loved Echoes, and my favorite part was the fact that you controlled two separate armies. I would really enjoy a game with two Avatars, each leading his/her own army. Twins, maybe?
3) Variable recruiting. One of the things that makes replaying FE with different teams difficult is that you always have the same characters for the same chapters. So, and this is my most radical idea, I'd like to see that change. Imagine this. There are two armies, one led by a male avatar and one by his female twin. Aside from a small group of core characters for each army, there is no recruiting done in story missions- only in paralogues. While the male and female avatars have their own separate story missions, like Alm and Celica, they share paralogues. Therefore, you must choose which army each character will be assigned to. They have unique supports with each army, and maybe even different promotion paths. For example, a cavalier sent to one army may become a Great Knight, while the cavalier sent to the other army becomes a Paladin. I, for one, think this would lend tremendous replayability to FE.