The minimal seal dancing we do now with SoT/SoR already feels super tedious and they're not even attached to any maintenance buffs yet. I can't imagine making it any more fun
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The ONLY way I could see seal swapping work is if not only are they on the GCD, but all seals also share a seal-unique 15s CD. As to prevent multiple buff stacking from being effective. This concept still works better with seals off GCD than on.
edit: just caught up on thread a little. maybe a potential way seals ON GCD could work is if it cleared all our HP? But that wouldn't even fix the problem it just means you would have to time your seals so you had 0 HP when you wanted to swap.