I found this project just recently, and I joined into the forum just to thank you for your efftorts, I'm not programmer at all so I can't really help you, but Aerith is one of my all favorite characters from all the final fantasy saga, and I have been waiting for a way to have her back at Cloud's side ever since I finished the game a long time ago.
Just a little suggection I'm no sure if this fits in your project, but I tought you may consider it.
The way I understood the game, and her later apparitions in the advent children movie, Aerith is not completely "Dead" I mean she can still have a large effect in the phisical world, she can actually "speak" to cloud, she saves him right after his final match with sephiroth (the famous hand sequense) aid Cloud in his battle with bahamut sin, and restore his life after he is nearly killed in the last confrontation of the movie againts sephiroth's brothers, in fact I susspect she became a even more powerfull magician after her "dead", well my point is that maybe (just maybe, please don't flame me on this one hardcore fans) aerith have all of it planned ahead, went Cloud and Aerith meet for the first time, she comments that she can't use her white materia, and she seem to know little about her Cetra/Ancient heritage, but after they visit the forgotten capital, she seems to learn a lot about it from the voices of her ancestors, maybe, again just maybe, one of the things that she learned & didn't told to anyone, is that to overcome her phisical limitations and attaint enought power like to manipulate "Holy" she needed to die, this will give the whole matter a most compeling insight, more than a ressurect, to undone a untimely and tragic death, it will be a rebirth, to complete a painfull yet heroic procces that will awaken her full power as a guardian of the planet, I tought it will be nice if you could ilustrate this point of view in her dialoges of later in the history, I mean that way she won't be talking just to fill in and make her fell real, but also to tell a compeling history about herself, here is a little example of how it could work
Aerith:
At first I was really scared of the tought of dying, but then I tought about you (Cloud) and I know you will help me come back, I know you won't leave me.
(Sometime after getting back at the Party)
Aerith:
I made a journey thought the lifestream itself in order to attain wath I need to confront him (Sephiroth)
(When getting close to the final battle)
These are just a couple of quick examples, but I think you get the idea.
A just a last thing, will be nice if some of her dialogues involve a lil flirting with old Cloud, I always loved when she did that.
Well that is all I have to say for now, Thanks again and don't give up, I know there are a lot of fans waiting to see this project reach its full complection.