It's probly been suggested before, but I'd enjoy it if a portion of mitigation came from self buffs for completing combos. Samurai, Monk, and Dragoon all have a portion of their play focused on insuring self-generated buffs don't expire and how they gain those buffs ties into the mechanics of their gameplay (identity).
I'm not talking dramatic here, but balanced in a way that cool downs are still used decisively. The idea would be that untrained tanks could still accomplish the casual content, but keeping the buffs up gives the healer more of a break. It'd open things up to a bit of diversity, like PLD combo-ing for a heal strength buff, DRK combo-ing for a small regen effect, WAR stacking max health buffs.
Not proposing very seriously here. As a new player, my progression from newbie to several lvl80's just felt I got some enjoyment out of hitting those DPS rotations to make sure my damage ups and skill speed ups stayed applied and it was part of feeling skilled and optimized as a DPS, but the only similar experience for tank is making sure you aren't animation locked to pop cool downs for mitigating busters.