I'd assume most people are asking for a Cossack Hetmanate, which is the genesis of modern Ukrainian identity. As much as I'd love to see a Rus' civ, it seems very unlikely when both Russia and Ukraine (legitimately) claim it. Personally, I can think of a dozen Central, Eastern, and Southeastern European civs I'd rather see in the game than the Cossack Hetmanate, which IMO would be pointless political pandering that doesn't actually help anyone or affect anything in the real world but...

NB I'm not saying the Cossack Hetmanate doesn't
deserve to be in the game; it would just be far less interesting to me than any of Bulgaria, Croatia, Serbia, Czechia, Romania/Transylvania/Wallachia, Lithuania, etc.