shamelessly stealing a feature from Disgaea
Every class has a "Counter" stat that allows it to retaliate based on how high the number is (per phase)
so a mage with a bad counter stat of 1 can only retaliate against 1 enemy and will just become a punching bag if a second enemy starts hitting him
whereas a Nekomata with a good counter stat of 5 can retaliate against 5 enemies in one phase, or because this is disgaea you can retaliate against the same enemy 5 times for a counter-counter-counter-counter-counter (but then you lose all your counters if another enemy shows up)
It would probably take some getting used to, but if Fire Emblem had a counter stat, then you could balance high movement classes by giving them low counter stats, so a cavalier with 8 move and 1 counter can move twice as far as an armor but only kill one person on enemy phase, whereas an armor with 4 move and 5 counter can at the very least damage or kill everything that tries to hit it. Of course you could use this against enemies too, so if there's an asshole pre-promote that's too scary to fight, you could try and soften its counters up with your good units, and then finish it off with squishy units that would instantly die if they could be retaliated against
This also helps solve the issue in defend maps where your hector is just too damn good at killing things and gets chipped away by 20 enemies because he keeps killing the body blocking enemes that are actually helping him. This also helps solve the issue of Seth solo-ing the entire map, where if you only use seth and he can only kill 1 enemy at a time, he'll quickly get surrounded on all 4 sides by enemies and chipped to death instead of just killing everything instantly with a javelin on enemy phase. So you actually have to use several units pushing forward together in order to bumrush maps.
I think this is a good idea, maybe don't keep the counter-counter-counter-counter-counter of disgaea, but the counter stat on paper is a good solution