Awakening: Remove supports/plot relevance from the spotpass characters, make them all-around more like the bonuses from FE8 (also make yen'fay and gangrel more unique), make Priam less lame, take Valm and make it its own game, give Chrom and Lucina more in-story development, make the story focus less on MU.
Radiant Dawn: Fix supports, make part 3 less "Ride of the prepromotes", give the dawn brigade more tier 2 time, make the script line up with the japanese hard mode script.
Path of Radiance: Promotion branches, ala sacred stones. Berserker Boyd, anyone?
Sacred Stones: More complex development for the lords, Eirika especially but also Ephraim. Make the last unit in the game not...Syrene. Creature Campaign-only level resetter.
Blazing Sword: Fix the whole "blow the plot load at chapter 20" nonsense, make Eliwood more lance-focused, make the Sol Katti stronger, add a non-crutch knight.
Binding Blade: Give Roy a Heaven Seal before the second route split, add dismounting (CECILIA), make the regalia stupid durable (Did you know that Roland slaughtered half of Spain, attempted to smash Durendal on a rock, and it still didn't break?), let Hector live as an endgame prepromote that isn't a magic user or Karel, make the plot line up better with FE7.