Regarding Roy, I think one thing that can definitely be improved is his air game, and I think he deserves at least a few new aerial attacks. Here's a few ideas that I have for his Nair and Dair.
Neutral Air: Burning Somersault: Roy spins in the air with the Sword of Seals burning, dealing fire damage when his sword hits along the hilt. I guess you could say it comes from a Mercenary's critical hit animation from games like Awakening, and part of Ike's Aether.
Down Air: Flaming Punishment: This attack starts off just like Roy's Melee Nair, with a fast sword swipe, but for the second sword swipe, Roy brings the Sword of Seals into his opponent from above, dealing a flaming meteor smash.
Yeah, that's as much as I can think of now.