Fates reclassing - I like it because any one can be anything, but you gotta work for that. you gotta put some work to make Azura a berserker, and doing stupid shit like that is fine by me. besides, it's a single player game, don't want to reclass? Don't do it.
Skill Capacity or Skill slots - makes sense to me. Only hold a certain amount of skills
Hybrid classes - I know not a lot of people like these, but in fates, some characters actually have decent strength and magic growths to make it work in hybrid classes. Azura has a 25% magic growth, which could be useable. Sakura is pretty fucking strong for a healer at a 30% strength growth. Saizo is running around with a 50% str, 45% mag growth. Scarlet and Oboro have a 20% magic growth. Not much compared to magic units, but it's something.
Dual guard/attack stance - only in this case, the only stat dual guard gives is movement depending on the class
A proper avatar customization - let me customize the class god damn it I'm also a sucker for customization anywhere
Different versions of the same weapon - Like the sword/katana
fates daggers - but only useable at 2 range
no weapon durability - I like the debuffs. Sure, the weapon system isn't perfect, but this is the second time out of 14 games they've gone without durability. And the first time was FE2
Light magic - Except it buffs the users and/or allies after cast
Dark magic - Not so straight forward spells, or spells that have an affect. nosferatu, or maybe a knock-off imhullu which increases def and res by 5 when user initiates attack.
Combine the magic triangle with the weapon triangle - The magic triangle was cool and all, but it hardly mattered because magic units had high res, so the damage was minimal. Sword/anima > Axes/dark > Lances/light or something like that
Controllable Allies - More in the sense of PoR and RD where you can give commands to the allies, not directly control them
Home Castle/MyCastle - A hub world where you can go about and organize yourself. Buy items, sell shit, promote, access DLC or whatever, view supports, yada yada yada
Tellius cav promotion - Cavs can choose their side-arm
Branching promotion - might be a doosy when combined with reclassing, but i've thought about an idea for it.
GBA + Fates weapon caps - In the GBA games, you can choose which weapon to S-rank whereas in fates you didn't have a choice. Leave the S-ranks to the mono-weapon classes, but allow the player to choose whether they want A axes B swords or vice versa
Limited supports - not in the GBA 5 supports per characters, but characters can only support with a small amount of people.
Gay S-supports - you know... for the rest of the people in the community that came out a bit weird
Tellius cav promotion not limited to cavs - but also the knights, maybe pagasi, and maybe wyverns.