Is it possible to heal friendly units that are damaged I wonder? Like a shaman/ healer unit. I think that would be a cool edition. A support healer unit. Very fragile and expensive. To balance it out, you could limit it so that each unit can only receive one stack of heals per turn. You could create a unique healer based on the theme of each army. It would work beautifully.
I would create a campaign map... where you fight against the Forrest theme armies.... and they would hide their healer units and archers in the woods. Those units would support their army and you would have to tactically take them out in order to defeat their defenses and push through. You would have to utilize Air units and ranged effectively.
About hide units in the woods, it can be done with fog war. And a hearler maybe can broke special abilities from some comanders
A healer unit? IDK about that, I'm sure that there is going to be unit-combining. But just an idea, maybe something similar to the black boat from AW Dual Strike, where it can repair units by 1HP, if any higher it would be broken.
Mercia is already able to heal so I'd imagine that it would be quite simple to make a mod to create something similar to a healer. As for an official healer/shaman/medic unit I'd be happy seeing something similar to that however the only issue is that you'd most likely find that people would just build clusters with healers in the middle/at the back. This wouldn't really make people use different strategies, you would just see the same thing every game. If healers were added they would have to be thoroughly tested and would most likely need a LOT of tweaks in the first few weeks of release just to balance them.