I'd rather have personal limit breaks interact with each other. Make a system similar to FFXI skill chains. All the LBs can be used separately, but if you time them together you get an additional effect. That additional effect creates a few second window for a mage LB to be more potent. Dragoon uses his LB, Paladin uses his LB, Monk uses his LB, puts an effect on the boss that increases the potency of the black mage's LB.
Each job could have more than one LB also. For instance, Ninja could have a ninjutsu LB that could benefit from the additional potency and a physical LB that can used in the chain. Dragoon could have a pure damage LB and an LB that does less damage but grants a crit buff or something.
Additionally, the LBs would interact with each other differently. A monk using his pure damage LB with the dragoon's pure damage LB wouldn't create an additional effect to boost the potency of a magic LB, but a monk using his pure damage LB with the dragoon's crit buff LB would.
