Disclosure: I'm a Russian nationalist and I'm biased but honest.
Putin had a chance to take most of Ukraine back or at least try to do it. This is late '14, Ukrainians taking hold on separatist "people's republics" of Donetsk and Lugansk.
They use artillery against populated areas, lots of civian casualties. The memory of Odessa massacre is still vivid. He has broad support back home to end it all. He has grassroot support in Kharkov and Odessa regions (connect them on the map and see how much of Ukraine is left). He's still holding arguably legitimate Ukrainian president for god's sake!
Ukraine army was still weak and disoriented, he had a chance of dealing a massive blow and overrunning most of the country, and he blew it!
Now, fast forward two years. Can we seriously talk about Putin expanding into Eastern Europe proper if he's unable to take Russian-speaking parts of Ukraine? How he would ever do that? Belarus wipes its feet with Putin, EU sanctions against it are all lifted. Kazakhstan improves ties with China and gets wary of Putin too. In the end he sublimates the inability to tackle Ukraine in his petty TV war in Syria.
Yes, he does use military threats, sending in military forces, trying to undermine democratic governments through supporting extreme left and right wing parties, and a massive internet trolling campaign. No, that won't get him very far, considering he's old and didn't have balls to take a bite at Ukraine. Perhaps he never wanted to. Of course he's bitter about Russia's decline, but he likes his little throne and oligarch friends and doesn't want to risk all that.
Summary: We can only talk about Putin expanding into Eastern Europe when he figures out Ukraine. Not earlier.