Hello Madrona, et al. Long time player, sometime doner. I'm here to make a case for healing, and how it could be made successful despite failing in the original game.
The only healing mechanic in the original game was the nanopod function in battle, where players could spend 1 nano and a turn to heal a unit; if I remember correctly, by 100 HP. This mechanic is so obscure and unused, most people don't seem to remember it, and as far as I can tell it's not even recorded on the wiki. It's difficult to imagine a situation where it is better to heal than attack with a different unit or retreating, as there was a good chance of the healed unit getting damaged again. And with the real-money cost, it's good that it hasn't returned in the Madrona era.
The absence of healing is sorely felt. Besides the PFP everyone is familiar with, medics were something z2Live toyed with per promotional material and old leaked videos of Battle Nations 2, but never ended up implementing into the live game. The following are my ideas for how to re-implement healing units into the game, keep them balanced, and how to construct quests and structures around them:
Healing units should have AOE heals
The easiest, most unobtrusive way to add healing mechanics into battle without fundamentally altering the turn economy is to providing healing units with AOE effects. If medics can only heal one unit at a time, this means that, like the nanopod spending in the original game, it will rarely make sense to heal rather than to attack or retreat. If healing is AOE, it will allow healing to exceed the pace at which units are damaged by the enemy, making it worth it for the player to bring them to battle. If they are to be viable, single target healing abilities should be much more powerful, capable of healing a heavily damaged unit nearly to full. I hazard this would be more difficult to balance however, and may be from a higher tier of medic unit.