I think the "best" way would be what Shurrikan mentions with more synergy within the kits.
It does not have to be combos for the sake of pressing 2-3 buttons rather than one, but rather "casting X amount of DPS spells charges Y amount of healing resources" or "using this DPS sequence within that window is somehow beneficial to some healing option". Any enemy attacking a tank with your regen would take some bits of damage. A shield on a target could be detonated as DPS on enemies if it is not consumed... that kind of stuff.

There are shy steps in that direction with systems like Earthly Star or the WHM lillies, but there is still much potential. I think the devs are afraid this would be too complicated to grasp or intimidating for casual / new healers, but honestly it does not have to be that way at all.