I would have loved there to be some form of spin-off game for him.

Also, I like how he was handled in the game - I'm sure when most people were introduced to the character they were no doubt expecting him to be a simple talking animal character who was just "there", instead he ended up being quite a key character to the storyline, and also one with a very serious backstory. In fact one magazine I used to have (lost it years ago, can't even remember which on it was) contained an early look at FFVII while it was still in development, and it talked about the characters and what was known about them at the time from the test version they were given - it stated that there was no obvious way Red XIII seemed to fit in to the storyline, and they assumed he probably didn't.