So here are some that I think would fit

1. Consider adding armour multiplier to fatigue (not negative like in vanilla but positive)
Something like scaling from 160% at fresh towards 100% (normal value) at exhausted.

Might help make the battles longer. Compatibility is a plus too. Though it won't affect magic damage but I dont think its that big of a deal. And I think it makes sense, a fresh soldier is far better at protecting his armour's weak spots than an exhausted one.

2. Same as above approach but with some other stats. Though what you can do is limited, again, compatibility.

3. Change towers to have wider fire-arc but shorter range, lower the wall/tower hp slightly, tie their hp to wall level in the settlement too (and faction, dwarven ones should be tough). Increase gate hp massively (like 3-4x, they go down far too fast). Experiment around with siege equipment hp too so it's useful. You can also change tower dmg but thats more troublesome.

4. Try to keep units generally on the vanilla level. Fiddle with stats if you must but do so in the way that doesnt make them over/underpowered compared to the original. That way people will be able to combine your mod with unit-adding ones at least, without feeling that something is wrong. At least not too wrong.

Those are only suggestions obviously, I just feel that you should stick to general changes. But make many of them. The more you make, the higher the (miniscule) chance that people will pick your mod over overhauls like SFO or new units.