Why does nobody see the easy solution here? Have the tactician accidentally fall into a Deeprealm or into a wormhole or something for FE6.
Anyway, I'd prefer if certain characters having certain stats let characters live. Like, for example, a maxed-out Hector would cause Hector to be injured, but not dead or something. I'd also like a Lillina Mode, but that might be too much. Oh, and, ofc, Lyn having her own kid. I'd replace characters like Dorothy with expies of popular Fates characters, because FE6 had a lot of really boring characters.
The support system should be a hybrid of Fates's (infinite support levels, love confession S-supports) and the original's (limited pairings, pure friendship endings). As for gay options for the avatar, I think all the lords, Legault, Florina, and some other girl at least should be marriable.