With regards to creating an attack force:
If that's too good it could also be done so that provinces in enemy dominion got unrest per turn. Maybe even scripted so that, say if unrest = 300 -(10xdominion) + 1 per point of pd + 1 per unit stationed there, the province flips automatically. Without the province flipping trigger, it basically puts an income penalty on people that expand a lot beyond their dominion, and doesn't even add much micromanagement, because of the new auto-tax system. With the province flipping part, unrest causing spells, and spies become (more) powerfull weapons for people with really strong dominion. The above unrest formula is actually pretty lenient, you could make it alot worse (you need 200 unrest, even with dominion -10, for the province to flip) Good vs Mictlan though.
There's probably something glaringly wrong with that forumula, but I can't see it. The actual numbers would need tweaking of course, but I think it might work in concept.