i would like as the OP says a limit break - overdrive system that only that job may use, if you combine that with tp would'nt be the same as the OP says, an abbility that deals 10 times more damage than a regular TP move like Cloud's limit break, Squall limit break, something that you could call kinda 2hr abbility, the only way i feel this system could work if you want to combine it with TP is to make it more powerful when you have full TP, so the ppl would wait the right time to use it more effectively, and the bar could be fill in a time radio (doing, receiving damage or healing) of 1 hr would be great specially if a dungeon content is added so you can only use it once in boss battles.

So that way, every job will have its own special taste, and will be important to upgrade yourself from a class to a certain job, and it could have certain levels for different limit break abbilities unlockable by ranking up and really, really hard quest on the respective job with cutscenes and all.