Here are some wacky theories I've heard that claim to (SPOILER)revive Aeris.
1. Go to Tifa's house and play Aeris' Theme on the piano 10 times. That will turn on a fountain in the pond at the City of the Ancients where Aeris' body is. Then if you go to the Northern Crater you can find the White Materia. Bring it to the fountain and Aeris will rise from her watery grave! (I heard this from a friend, I think he made it up and tried to pass it off to me as real but he won't admit it)
2. First, Master a Phoenix Materia. Go to the City of the Ancients and equip Cloud with his Buster Sword and place the Phoenix Materia in the first slot. Place another unmastered Materia in the second slot. If you go to the place where Aeris is buried, suddenly you will be able to Summon Phoenix outside of battle. Summon him and he will unite with the spirit of Zack (I'm guessing that's why you need the Buster Sword?) to revive Aeris. When she comes back you will Master the Materia that you put in the second slot. Supposedly this only works in the Japanese version. (this I found on the internet, I lost the source)
3. Beat the game 5 times without turning the PlayStation off (is this even possible?!) and when you win the fifth time a game will save in the Temple of the Ancients with a different storyline where Aeris does not die. It said that this was only in the International Version. (I forget where I heard this, I think someone emailed it to me)
4. I forget exactly what it is, but there's supposedly something you can do while on the date with Aeris that will either prevent her death or cause her to rise after Jenova-LIFE is defeated. (I think this was on a message board somewhere, might have been here)
Where do people come up with these things?! Anyone else heard some wacky (SPOILER)Aeris resurrection theories?
~MJE~