I'd love it if healers got redesigned to be fun again, but that's definitely not happening in a mid-expansion patch.

Healer balance is fine. Scholar doesn't need damage potency bumps. I'm hesitant about AST's MP economy too. While the MP struggles are something I remember well from late Heavensward WHM, I'm also keeping in mind two things: 1) AST is currently the numerically strongest of the three healers in both healing AND damage, and 2) improving their MP economy will increase those numbers. Not directly, but also remember that having another MP restoring tool means that ASTs won't have to meld as much piety, which frees up slots for materia that increase their damage and healing potencies. That's going to be the unintended consequence if the devs heed that request.

On that subject, I'm also iffy about MP management being a "good" way to increase gameplay interactivity. At least not in this game, and not in the way the kits are designed. I get it, I've played other games where keeping on top of your MP was important and it certainly -can- improve engagement. I don't think it'll do it here, at least not in any significant way. Why do I think that? Because healer kits don't have a whole lot of meaningful choice in them. A big part of the fun in keeping an eye on your MP is managing tradeoffs. Do I cast this heal because I need it, or can I get away with this other one because it's weaker but cheaper and I can keep myself rolling longer? It's a game of managing your resources carefully enough that you can keep up Always Be Casting, because your punishment for failure is running out and needing to stop the spells for a bit. FF14 healer kits have almost none of this save something like Cure vs Cure 2. I remember bleeding mana at the end of Heavensward as a WHM; that wasn't a game of "careful MP management", it was a game of realizing that my spells cost way too much MP and I literally couldn't indefinitely follow the ABC rule performing my basic tasks because there wasn't a choice I could have made that would keep me in the game if I played correctly. Just keeping up my basic lowest-cost rotations would eventually bleed me dry no matter how I spun it.

The point of my rambling here is, MP management is a good mechanic if you have a reward (you get to keep ABC) and a punishment (you have to sit for a bit to get your mana back). With the way healer kits work now, you don't really have much control over your MP output aside from never touching GCD heals and not raising people. And I definitely wouldn't want to be so MP negative that healer downtime, boring as it is now, is reduced to sitting around waiting for MP ticks.