My concept for Ultimate Skills in FFXIV WIP

[XiroGear]

Please keep in mind that this is a WORK IN PROGRESS, what is written here is not represented of the final Idea.
Ultimate skills Are powerful (where normal skills are concerned) And Flashy skills that Can only be used sparsely And when certain conditions are met.
Potency: 400
Caster potency: 200 per target
Cooldown: 5-10 minute GCD shared for ALL Ultimate skills
Cast time: 30 sec-2 minutes
Ultimate gauge(?): use a charge skill to channel 100% Max MP into gauge (If, for example, you have 9000 max MP you must channel 9000 MP into the gauge to fill it completely even if you have under 9000 at the time, Example: If you have Max MP of 9000 but only have 6000 when you start channeling you would still need yo channel 3000 MP in order to fill the gauge completely), will be immobile or unable to use other skills until charge is complete or MP is fully drained, using other skill will cancel charge. Whether or not you lose gauge when you break channel is unknown.
Multiple Ultimate's: if one is used the rest will get locked by UGCD (Ultimate Global Cooldown)

Please let me know what you think. I want this to be usable but not abuse-able, useful but not broken.