Another theory being thrown around is that Putin also wants to reduce Russia's western defensive line by reducing it down to 600 km versus the 2000+ km it currently has after the break up of the Soviet Union. By taking non-nato nations on the European side, its back to the original line before the pre-soviet collapse.
That is just the same theory expressed in a different way. He cant control the people of Ukraine by giving then a decent life as a Russian satelite, so he's invading them to force a pro-Russian government on them.
