I've floated this idea a few times to make spears and pikes a bit more viable without just heaping more stats on them. So here is the idea.
Spears and Pikes gain Common Soldier promotion. This upgrade is LOST on promotion to Tercio (tercio are just fine and don't need any help).
Common Soldier: Heals 5 at the start of its turn, even if it takes an action (not effected by medic).
Mechanically this is a "mini-march", but most importantly does not stack with any medic promotions (which I need to check if that is mechanically feasible). The thematic is, because these are the common cannon fodder, you have tons of these people compared to your more specialized and well trained military units. Aka where 1 man falls, another takes their place. And so they recover a little bit of health on every turn if they aren't destroyed. This gives them more staying power on defense, and makes attacking with them a bit more viable (as long as they aren't just destroyed before they can heal).