Right, you got my main point, and I think it can work better on a fantasy RPG like this one because the combat isn't going to be as fast as a shooting combat like Borderlands or Mass Effect sagas.
Some MMOs tend to create lot of skills for each class, and at the end, players just use like the same 4-5 skills, and with a system like this all those "extra" Skills can be useful.
Imagine being a Mage with Arcane, Fire and Ice skills, you can have different set up against each type of bar.
If you have to fight a boss that can change between bars by conjuring magic Shields or more Armor, you will have to change between rotations, instead of just using the same rotation forever.
I mean, at the end repetition is part of the games, but I think 3 different rotation depending on your enemy are funnier than 1.
Totally agree, you cant have infinite original bosses but your gameplay can be, by rotations.
Love your idea!