Ok, I looked at FitztheRuke's post. The area I didn't see addressed as a dial was action economy. I would look at Bonus Actions and Immediate Actions as areas that could do some work here.
Ok, so your statement of "For me, if you want to really privilege in-combat healing more, I think the first step is to dramatically lower the ability of characters to have innate and massive out-of-combat healing" wasn't an axiom about design possibilities within D&D or TTRPGs generally, it was about 5e in particular with respect to your perception of (a) the consequences to 5e's balance and (b) the workload required to rejigger that balance?
Have I got you right?
Yes. It was a comment about 5e specifically, and about (from my perspective) the most likely means to privilege in-combat healing.
This is with the assumption that by "in-combat healing" we are talking about the use of spells and other class abilities to allow in-combat healing, and not simply adopting addition "surge rules" as in the optional rules within the DMG.
Can you maybe talk more about 3 things:
1) What are you referring to by balance here. Are you talking about some notion of class/build parity (qualitative or quantitative)? Are you talking about niche protection? Are you talking about table time allotment and/or the impact on the arc of combat (eg more consequential in-combat healing will yield a "rally arc" and/or increase table time on combat)? (Are you talking about Adventuring Day pacing (already a fraught subject in 5e)? Are you talking about the impact on orthodox D&D scenarios (particularly Team PC vs Obstacles and climax vs anticlimax)?
Of the topics mentioned, I would lean toward the notion that it would disrupt the resource economy in 5e, particularly as it relates to combat.
I don't think class/build parity would matter much, tbh, and I don't think niche protection is much of a thing in 5e at this point.
Where I do see the issue is that 5e is still tied to the resource economy of the adventure day; while the short rest mechanic breaks that up somewhat, it appears that they are moving away from that in more recent books. So I would put this under the rubric of the "adventuring day." The combats mostly rely on iterations of an attrition model, and tension in combat (to the extent it is generated in 5e) would be dissipated by increasing the amount of healing to equal the amount of damage during combat.
As it is, the tension/release is generated, for the most part, by "getting through" the combat and then healing up.
2) After you've laid out your concerns for 5e balance in (1), what is precisely your fear (outside of the overhead required to rejigger the numbers/align resource suites) of making in-combat healing more consequential? For instance, what sort of paradigm shift would emerge from someone hacking 5e's engine in such a way. Is it simply that the drift toward 4e would be so substantial (your last sentence of your 2nd paragraph seems to imply that) that you may as well just play 4e? What if someone says to that "I don't want to deal with the AEDU resource scheme, all the Forced Movement, all the Terrain/Hazards interaction, all the mobility incentives/requirements, all the diversity and synergy and scope of Team Monster + battlefield array....I just want to keep playing 5e yet have some legitimately consequential healing that I can deploy on a per encounter basis to (a) thematically change the shape of our play and (b) to stimulate me tactically...I think that is doable without a massive drift toward all that 4e entails."
I don't have any real concerns on that. If someone wants to put in the time and effort, good for them! That said, I think it would be a fair amount of time and effort for decreasing rewards. I think it would certainly be possible, but would involve (for example) changing other mechanics in fundamental ways- removing death saves (and/or whac-a-mole) or greatly increasing the power of monsters. If they do it, awesome sauce!
3) Do you think 5e is so meticulously balanced around your conception of (1) above that fiddling about with it is going to be House of Cards-ey? I feel like that would be a pretty controversial take so I'm not sure you feel that way but I'm curious given your response above.
Ooohhh. No. I don't think 5e is meticulously balanced. At all. Which is why you can screw around with it so much. In fact, I think that it would be quite possible to just say, by
fiat, that healing spells do more healing and have a go at it.
But ... I do think that this was a deliberate design choice, and because of the resource management and attrition model used, it will likely have more effects than first considered.