Third Rome DLC focuses on Russia. And only Russia as far as I know. Other DLCs would usually focus on either an in game mechanic and try to improve it while still adding other things on the side or they'd focus on a geographic location while still adding other things on the side but not here. In here its Russia and Russia only.
+New ranks for the Russian government.
+Commission religious icons for the Orthodox Church, giving you buffs.
+Special Russian units as well as unit models.
+New missions for Muscovy, Novgorod, Russia and other principalities.
If you really wanted to play as Russia then here you go!