This KH theory is very intriguing. It gives a greater depth to Ventus' character rather then just being the happier version of Roxas. It does debunk the reincarnation theory that's been going around the fandom for a few years but has such most have probably already mentioned that.
Why not take this theory even further. What if Ventus is the player character of Unchained X? We never notice because of the mechanics of the game. Being able to customize the player to his or her hearts content. I know it seems a bit far fetched but anything could happen in the kingdom hearts universe and it would be pretty mind blowing has well.