How about, Medics will be able to perform the usual task of heals but instant heals will only recover health moderately whilst the stronger heals are Heal over Time (and I am not talking about ten seconds). Aside from this, healing skills will require the use of 'prescriptions' (couldn't find the ideal word) which are basically crafted from medicinal herbs or involve difficult processes of extractions that only the most experienced Medics can accomplish. For all I care, the plants in the wild won't have any names and you will have to depend on their looks, and there may be similar looking poisonous plants. Back to what I was saying, these 'prescriptions' could be carried in a separate Medical Kit and once exhausted you can no longer use your heals. Perhaps you could even have caravans bringing medical supplies to your camp. Or even better, over-healing will trigger off the side effects of the plants in a way similar to how poison can be used to neutralize other poisons (according to Traditional Chinese medicine anyway)
In a way this would put it at a cross between Crafter/Support but I suppose you could choose whether to specialize in the Crafting aspect (Apothecary) meaning you produce better quality 'prescriptions' or in the Support archetype (Healer) which will give you bonus stats that will help you survive on the battlefield preferably in terms of elemental resistances and whatnot, or even follow the same line as above to manufacture Poison Kits.
I can see how the involvement of Crafting might be a serious flaw, but does this concept sound like it would work in principle? Doesn't hurt to dream I suppose.