A secondary TP bar could work, one that cools down 1-2 points per min but only goes up: (an takes about 2.5k to fill)
When your hit 2-4 points for a basic attack 5-10 when your hit with a WS
When you attack 1-2 points AA/Basic 3-6 points WS

So in real world terms you would rarly get to use it unless your on a very long grind (so could be used as a oh s*** save if you keep it)
But in some areas (like the bowl of Embers) You enter with 0 but it fills at a rate of +100-200% of the above numbers, meaning there is a chance you can use it but its still not easy to get.

As for the skill itself, id love something involved (a mini game) not just a 1 click pony attack, where you can build it up or fail to build it up and have each class/job do it differently..
I would set a class LB at ~2-3x the damage of a WS, and the Job LB at ~3-5x personally
(maxing at 6x and 10x respectively with best reustls from the mini game thing)