The thing that I would especially love would be some gaiden chapters featuring the characters who don't have a lot of screentime. Just think of how awesome it would be if you had a paralogue with Tormod, Vika and Muarim or even a paralogue explaining how Geoffrey learned of Renning where you team up with Batsian and Volke. Or my personal favorite: have defend chapters between the tower chapters where you have to stop reinforcments from entering the tower of guidance and failure would increase the difficulty of the next chapter. This sould force the player to keep some good units out of the tower.
Also for @Anacybele, I have an idea of a potential "Shared" ending between Ike and Elincia that wouldn't be out of character: "As the war ended Ike left Tellius never to return, and Elincia kept her duties as queen. No matter how much time would pass, or the distance that separated the two they still thought of each other. One day, when Crimea was fully repaired, Elincia dissapeared, rumors state that she was seen at an harbor shortly after."