I don't really see the point in punishing players for using a single, quick X potency heal without having first used the slower X+Y potency heal. That seems like that's just devalue ability to estimate incoming damage and C3's capacity to, say, heal a weak but short White Hole mechanic without needing oGCDs (so long as the team stacks). I don't want heal combos or arbitrary need to save something for after the very thing most likely to make it redundant.
If the problem is button count, then just allowing Medica (and perhaps M2) to center on others and deal double the initial (or an additional 400p) healing to the target and those within n yalms seems a plenty iconic capacity already, saving just as many keys without the loss of choice in timing.
Meanwhile, I'd go the opposite way for M2, reducing the initial potency and shuffling that into ticks, perhaps via some indirect benefit (say, overheal ticks increase maximum HP for the remainder of the original M2 cast's duration). That way there's still a measure of choice.
Else, if you really want a unique but button-dense approach, just extend off WHM's conjury theme and introduce some related traits that can spread and/or duplicate-over-time their overheating. Perhaps even allow them to convert the HoT to other things (say, by allowing Benison to absorb incoming periodic healing up to X% of target HP) and/or apply brief buffs to allies that would incentive bursting a heal off of one vs. another, etc. perhaps do so only off of crits, with some heals having higher crit chances than others or ramping their chance towards a guaranteed crit and giving a bonus to a(ny) other heal once they Crit, etc.
Heck, have healing potency done (perhaps taking the best recent per target cast) briefly increase your Offensive Magic Power so that you can seed a Regen or empowered Cure II just before a burst damage phase, while Wind spells in turn can increase access to early cast completions for healing spells or access to instant heals (say, by having each tick reduce Cure's cast time by .1 GCDs, etc, as not to need the Afflatus buttons and to be more distinct from SGE.