Aeo is on to something, making Aeris a 'party guest' type of figure would make a lot of sense and help all the peeps posting the hastag #dont-level-aeris :P
I kinda admired that she did fight though, and her limit breaks weren't half bad (mainly because they were irreplaceable). But...what's the point?
The point was that there was absolutely no indication that she was going to die. She was treated just like any other party member up until that point, which made her death much more shocking and meaningful.
I sincerely hope they do not
make her some sort of guest character. Sure her death isn't exactly a secret anymore, but I still feel like making her more like an NPC than a real party member would cheapen the experience.