You're ALL looking for the wrong solution. What FFXIV need is take a page from Wow and make tanks AND dps take damage from any and all enemies so they have to actually heal and be pro-healers. If healers had to really worry about people dying they'd have to actually learn to be great at healing. The healing ceiling isn't high enough outside of Savage raiding, that's the problem.

Where's the dungeon mob that hits dps like a truck?
Where's the dungeon mob that kills the tank in 4 hits?

I remember when Stormblood came out healing looked hella fun and tanking as well because it exposed who were the good tanks/healers and who needed to level up in that department. It didn't create a situation where dungeons couldn't be cleared but it created a situation where using ALL your tank/healer skills mattered. You actually had to spam more healing than just cureII, you needed to actually manage mp and manage your oGCDs.

So yes, make enemies more dangerous, it's fine if people die, they just need to strategize better and the healers will need to stop dpsing and use all their powerful healing skills that they're getting to their maximum potential. I just have to emphasize to the devs that just because you make healers heal more that doesn't mean that you'd be making the game stressful, you'd just be shifting the meta from dpsing all the time as a healer and throwing 1 cure here and there. Currently healers.....don't really heal, they can literally read a book and just press 1 healing button all day. The healer role needs to become a healer role again. Or just do what your'e doing with tanks and give them A LOT of support and damage skills....but tanks have that covered now so instead just focus the content on rewarding good healing, that requires enemies that will damage tanks hard and even the dps. I never get why dungeon mobs and bosses rarely if ever damage the dps, really weird.