we discussed this a while back before beta1 came out and i THINK, but don't hold me to this, i THINK a team member said a limit break or anger bar would be considered in later versions. Like you have said they could make it like a limit break in FF7 or I like to liken it to the limit breaks in Toshinden...
When you bar fills up you get, say 60 seconds, to perform a limit break move. It would be a melee combo with perhaps a ki beam/shot finisher. If your target blocks the initial charge that the attack does at them then it is wasted and you dont perform the move. Otherwise they get hit and suffer the whole combo.
Like you also said they could give characters some special things that would not otherwise be implemented like Goku's Kaoken (which could be his limit break while hes in original form).
The limit break bar could also be used in power struggles and melee struggles, so it should be a decent and worthwhile addition ot the game. (Someone even suggested that it help in performing a transformation earlier which makes sense!)