It'd be hard for me to just pick one, honestly, so I'll try to just list a few! All of these could be done with or without shipping, what with this being Fire Emblem and everything.

One thing I'm interested in exploring is what happens in the Crimson Flower route during the timeskip, especially with Edelgard having placed his father under house arrest and his conflicted feelings about that. Perhaps he and Hubert get into a particularly heated argument during a war council, and she feels compelled to try to sort things out with him.

I'm also a sucker for all the support conversations in the game where one person is overdoing it and someone else has to step in to take care of them. Even though Edelgard's pretty good about delegating responsibilities, maybe she takes on a little too much, and he tries to step in to help. Or it could be the other way around, where she notices that he's working himself to the bone and tries to admonish him.

This last one is purely sappy and self-indulgent. Since Ferdinand writes poetry in his free time, I think it'd be really amusing for Edelgard to discover that he's written a bunch of poems about her (the complete opposite of the pamphlet that he tried to distribute when they were younger), and him getting embarrassed about it.

I'm also down for any ideas you might have as well!