I think MCH should be similar to its PvP Counterpart, with the Battery Gauge being completely removed, and the Heat Gauge serving as a limiter for most Skills to not be spamable, but in exchange, most Skills are tied to the GCD and Skills like the Scattergun, Air Anchor, Drill and Auto-Crossbow, have instead multiple charges that dont naturally recharge, but instead get fully reloaded with a "Reload" Skill (not tied to GCD).
As for how to deal with the Heat Gauge, once the thing overheats, it will lock you out of most Skills, but unlock certain others in exchange which will serve as a "cooling method" for it. Those Skills which are unlocked this way, could be for example Flame Thrower/Bioblaster or Chainsaw/Heatblast.

Also, remove stupid Filler abilities like current Tactician (Healers should have more focus on mitigation from a Support perspective), Wildfire and Hypercharge which is just inner release in disguise, Heatshot and Gaussround/Ricoshet.

Unpopular opinion, but not every Job needs freaking oGCD's.