Well, if Russia only invaded Ukraine, I could see it turning nasty at some point.
While the occupation would be difficult enough, and assuming it had somehow managed to supress resistance, it could still spark some kind of WW3. Before the Russian army even marched on Kiev, the eastern states of NATO would become extremely jittery. Poland would not want an occupied Russia on its border and while a forward march into Poland would be unlikely, I think NATO's millitary capacity would be on high alert. As a previous poster mentioned, any NATO soldiers training in Ukraine could easily be caught up in the conflict-then what? Likely response would be the direct arming of Ukrainian insurrectionists, which would probably lead to Russian attacks on NATO supply lines. Accidental or "Accidental" strikes on NATO personel in Ukraine would probably continue, as well as surgical SAS strikes on high profile Russian millitary personel in Ukraine.
I think overtime, Russia's plan would be to ware down the nearby baltics with pro-Russian propaganda. Probably painting a picture of Russian occupied Ukraine being some sort of a safe haven. NATO would be expecting this of course, so with tensions running so high on both sides, I think there would be a general build up to some kind of conflict between NATO and Russia. Possibly a sudden massive air onslaught or a jittery Royal Navy in the North Sea launching attacks on a nearby Russian ship. Russian planes "buzzing" US ships would almost certainly see them being shot down.
It depends on how Germany would respond to a Russian invasion of Ukraine. In the book "War with Russia" NATO almost collapsed because the Germans found it difficult to abandon their pascifist stance. If it seems there was ongoing division in NATO and the EU regarding a response to Russia, it could provoke Russia to be more boistrious and aggressive if they believe they would be limited NATO resistance. Besides, as soon as it seems there is a whiff of imminent confrontation between Russia and the West, what would happen to the global economy? Surely gold prices will go through the roof in anticpation of a possible breakdown in oil infrastructre in Europe. If Polish and Baltic citizens become too jittery and begin moving away from the eastern borders, near possible Russian invasion areas, how would that affect their economies? And the eurozone economy as a whole? A growing economic crisis because of East-West tension could lead to public dissident, fertile ground pro-Russian propaganda.
In any case, NATO could respond by ramping up attacks against the Assad Government in Syria to destablise Russia's shpere of influence. If NATO could create another headache for Russia in Syria while they were dealing with Ukraine, it could overstretch Russia's millitary capacity. I think that indirectly, a Russian invasion of Ukraine in 2015 would eventually escalate into a short but nasty World War 3.