Why did Russia gain their huge land empire? In my theory, it was because they were the closest civilized peoples to the Urals. The Urals kept out population diffusion, trade and civilizing ideas and technology until Europe had developed such an advanced package of technology that they were able to break explosively across the Urals to conquer the Siberian wilderness.
So what if European Russia can't keep it together, and for whatever reason it remains like Germany through the centuries: a disorganized mess of small states, tsardoms and kingdoms? Combine this with maybe earlier German successes at unification, maybe for some reason a higher population... is it possible that the Germans could step up to become the society strong enough to conquer such a huge landmass?
If so, such a power would be nigh unstoppable. I would think that they'd partition up central and eastern Europe between themselves and the Ottoman Empire, keeping unruly populations down. We might just see a huge land empire expanding throughout Eurasia before they collapse due to nationalist and ideological forces.