The base idea of self ultimate skill can be interesting, but I'd also see this as a revamped LB system.

But, on top of that, it would be fun to bring back "Skillchains" but for those limit breaks with new animations.
You'd need two healers LB at the same time to Raise everybody, two tank LB to reduce damage by 50%, etc...

You could have a MNK kicking the BLM meteor like a Blitzball !