Playing as the Sylvan faction (Disciples 2).

adding a critical hit mechanic seems to work well

I still don't understand the sacred meaning of a critical hit. Is it a stable bonus that ignores armor? If so, it makes sense. Otherwise, you can simply increase their damage without any complications.

and it seems they can't attack different targets

The thing is, they can, if I remember correctly. But I often found it too tedious to build this branch, and they also consume a lot of resources. Plus, you need to upgrade the centaurs and the healer...

in the add-on, I encountered a race of dark elves

Unfortunately, you can't play as them, there are just a few units. And that's a pity. They are VERY powerful and hard to defeat, and they always caused me a lot of trouble.