I'm not sure about rewards, but I would love to see an event where the gang is all in the Gold Saucer. You have to help Cloud decide on whether to date Aerith, Tifa, or Barret. Then add in some new mini-games to the Gold Saucer that were found in the original game (snowboarding, submarine, boxing, etc.).

Also, I didn't even think of it but as others have mentioned it would be awesome to have the opening bombing mission as a dungeon. Maybe utilize the Trust system so you can enter alongside Cloud and Barret!