Slavs need a buff. A bear buff.

So the game has bears, which are just as strong as wolves. The slaves are okay but not amazing, like all of the Forgotten civs (or at least the ones that didn't get major reworks, which is all of them except the Slavs).

The solution is that Slavs should be able to train bears as cheap feudal trash units. In Castle age or Imp they could get elite bears. Because Russians actually used trained bears for stuff (the chinese did too, and the chinese also need a buff!)