Units: Have a somewhat expansive [or just go all out] unit creator. at least 40 hairstyles for each gender, at least 5 different body types per gender, at least 25 different face types per gender etc. Enemy units can be defaulted based on the game.
What games: From FE3 to FE15/FE16
Maps: Make maps customizable.
Weapons: Use all weapons from all games.
Story: Make an option to put in a multi-chapter game chain where you can save per chapter [and battle save if the creator will allow it] along with making it to where you can make a unique story with dialogue and somewhat unique characters or something.
Sprites for older games: Make them match the unit's looks.
Skills: Make it to where FE4 and up have skills.
Stats: Make stats completely customizable.
Classes: If a class' base max stats are modified, make it to where there are customizable class names in that situation.
Music: Make it to where ALL music from ALL fire emblem games is allowed. Not only that, but maybe update the music list to have songs from other games as requested, if possible.
Adding and removing units: Make it to where if a multi-chapter user created experience, for the user to force some units to be unusable or straight out removing them.
Clear objectives: Add all the clear objectives from all games.
Adjustable stat growth: Make this a thing. Simple as that.
Voice acting: Make 25 unique voices per gender at least, along with adding voices from previous fire emblem games.
is it a dumb idea is it a good idea i dunno. I feel like my ideas have holes usually. I'm probably missing something.