I like this idea a lot actually, very creative. My thought for how Bahamut would be that after you summon a primal to aid you at 100% it would give you a "Charge" or a "Gem" and once you have done so with all your summon bars you can summon the big B himself. This way it would force you to use all your summoned over the course of a dungeon/raid/encounter and when you final do summon him it feel really important and powerful. That is something I like about SUM right now is that you can't just walk around with Bahamut out all the time.

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