I think the Russians are pretty much at the limit of their available military capabilities in capturing Crimea. In my scenario for a Russian attempt to seize Crimea and eastern Ukraine, I assumed that Crimea would be the theater of war while eastern Ukraine would be limited to a road march for a relatively non-violent occupation of Kharkov and Donetsk.
Right now, Russia would have to fight if they advance into eastern Ukraine:
Ukrainian paratroopers with armored vehicles were digging in on Thursday near the Russian border as the military demonstrated its presence in the east following Kiev's loss of Crimea to Moscow's forces.
It (a company??) is deployed in the Donetsk region and is a unit of the 25th Parachute Brigade.
So there will be armed opposition, unless the Russians can bypass the unit on the way to Donetsk to support Spetsnaz-led uprisings. But Russia can't assume that Ukraine won't fight on a broader front within eastern Ukraine. Especially given that Kiev gave up Crimea without a fight. The government might not be able to survive a further surrender without fighting first.
And were I God of Ukraine's military, if Russia does try to take more of Ukraine, Ukrainian forces would reopen the Crimea front by trying to retake some Crimean territory and by trying to damage Sevastopol and the Russian fleet there.
Then Russia could be faced with a military struggle that is beyond their capacity to win--or at least beyond their capacity to win without making embarrassing mistakes that undermine their reputation as a great power.
If Russia thinks that achieving more against Ukraine right now is beyond their capacity, what might they try next?
Russia has made plenty of noise about owning territory in the Arctic Circle. So my first choice for Russian moves while NATO is looking at Ukraine and the Baltic States would be to stake out contested territory in the Arctic Circle.
Why risk pushing NATO too far over Ukraine when a larger struggle for Ukraine might be outside Russia's military ability? Go for something that won't resist--ice covered territory in the Arctic.