They could keep the cooldowns short but instead do a sort of point-based progression like Assault or Einherjar.

To prevent spamming, they could have GCs do the same thing as Assault Tags. If you have a tag you get points. But if you don't you can still go in but don't get points.

To avoid it being a "fatigue" debacle they could put in quests to get more tags in a window. Use your tags, do the quests to get more tags, whatever you don't use by the reset is lost. That way hardcore players aren't impeded too much.

The content should be cooperative, the progress to reward should be personal.