Excellent overview by Stratfor, illustrating the range of Russia's military options in eastern Ukraine. The last and most practical option Stratfor presents is the one I predicted immediately after Russia annexed Crimea in response to the Maidan coup. The Dnieper River provides the most natural and defensible border between Ukrainian and Russian interests, providing Russia will a broad and defensible land bridge to Crimea and its strategic warm water port of Sevastopol.
I was surprised that Moscow didn't go ahead and invade eastern Ukraine and get it over with last year, choosing instead to provide limited support to the separatists in only two oblasts. But that opportunity might present itself again; as soon as the muddy steppes dry out from the spring rains, I expect Kiev will launch another offensive to encircle Donetsk and reestablish the border with Russia. Moscow will cite Ukraine's violation of Minsk II as justification to pounce on Ukraine's extended army and to consolidate everything east of the Dnieper River. That's my prediction, but I guess we'll see...