for bringing back Jill, it's possible as playable character. but for Claire and Barry, that's inappropriate.
Claire's already been out from this outbreak virus hell hole, she's working on TerraSave, just to provide aid. for making her return as cameo, that's acceptable but if she's return as part of main character, that's stupid i'd must say. she's already did her part, so no need for her to return in the main role.
that same goes to Barry, he's too old and have family. it's dumb for his age to return in the main battlefield (wouldn't want him pointing a gun while holding his walking stick, no?). he ain't like Chris that can punch boulders / throw someone easily nor he doesn't have any special blood.
but the rest of the characters like Rebecca, that would be great. except for Steve, he's gone, he should to stay dead.
for Billy, he's too cool to return into the outbreak incident. his character should stay that way, long disappeared and started a new life.