MNK and DNC (if it were to be implemented as
I envisioned it) would instead go off a momentum system. Using PGL as an example, your TP bar initially goes from 0 to 100 when you get your first stack of Greased Lightning. Once that happens, the TP bar decays over time and subsequent skills restore some TP. Getting a second stack of Greased Lightning increases your TP to 200, with it decaying faster while your TP is between 101 and 200. Getting a third stack of Greased Lightning would increase your TP to max at 300, and it would decay even faster when your TP is between 201 and 300. If you fall below 300 TP then you lose your third stack of Greased Lightning and go back to 2 stacks. If you fall below 200 TP you lose your second stack of Greased Lightning. If you let your TP hit0 you lose your sole stack of Greased Lightning.
The bonuses for staying over 200 TP would be increased speed and damage and access to certain skills (thus making certain skills require a specific level of Greased Lightning). The bonus for staying over 100 TP would be just increased speed.
Note: I got the idea from the difficulty level gauge from the game Godhand. >.>