These aren't mechanics, these are just sort of things you can do if you talk it through with the GM, which is already really part of the game. This is all speculative and not at all playtested. I just decided that I need to start actually posting some of my partly formed ideas rather than having them sit in my notes folder forever. I don't plan to do anything with this. This is why I haven't even put it in Extra Stones. Maybe this is me more trying to train myself to think about rules-light mechanics more creatively.
Defending someone: You use your action to use your shield or weapon to defend someone else. You take the damage, and the attack is impaired, so long as it makes sense (like it probably would for a goblin, less so for a dragon). You need a shield, or a large weapon held in two hands, and can't be doing anything else.
Shield wall: You need a shield. It takes one turn to form one, or you can form one before combat starts. You have to move together slowly, as a unit, which means you can only attack someone directly in front of you, usually someone who moves to attack the shield wall. Attacks against the shield wall are impaired so long as it makes sense in the fiction. A shield wall is broken if anyone in the wall loses a morale save, dies, or takes critical damage, or someone gets behind the wall.
You have a bow, and they know it, and they do not have a bow, and you're aiming it at an open space.
The first person to cross that open space (and be hit by an arrow) must make a WIL save to do so.
Actually, much like for morale, I'd probably let the PCs do whatever bad idea they decide to do, and just apply this to NPCs.
You must be riding a horse trained for war. A human-ish creature must make a WIL save to stand their ground and attack. This can also break a shield wall, if someone doesn't stand their ground.
Flanking This is exactly the same as when you both attack at the same time and take the higher value, but what happens is one of you has distracted them so the other can land a hit. It's just another way of thinking of it.
Sneak attack If you're able to spend a round, not in melee, moving around behind the enemy, you can deal enhanced damage the next turn. Unless conditions are particularly favourable for this, it might need a DEX save.