Ooh, good topic. This is fun to imagine. I'd definitely like them for the characters where there's some mystery or debate about how exactly they died, like Kyouka, Hisame, Ryokan, Sakuya, Shigeto, Shoji Katagiri, Chitose, Ayako, maybe Madoka. Ayako especially would be great, because I'd love to get more hints about whether she really is the last survivor or not. Although FF5-style glancing doesn't necessarily show the actual death scene, so it's possible it might not resolve much.
I'd also love one for Kageri Sendou! I bet her final walk with Watashi could have the kind of peaceful+disturbing vibe they like to go for. And Kureha would be brilliant as well. Just to see what she was like when she was alive.
I know what you mean about finding it hard to imagine the series without it. I think the idea has always been there for the important ghosts, at least - there's always a big build-up to seeing that vision of what happened to them and the ensuing disaster, so the form it took in FF5 just feels like expanding something that already fit in well, rather than pushing an idea in where it doesn't belong. I hope it's something they keep in future, though I'd like a full-length ghost list too.
ETA: Ooh, and Long Arms! Just to finally get some clues about what was going on there.