you have sometimes just to look at a map like a military planner or an imperialistic ruler would look to see the obvious
Estonia can only be the next victim because it has a long border with Russia and if you take or destabilize Estonia you can have enough influence on Latvia and maybe Lithuania. This is why Estonia is quickly becoming the next point of attention (and of preparation in case if something starts happening). You also have to consider that they have a big minority of Russians and that there are already patriotic Russian seperatists active for a long time.
Ukraine : well if Ukraine is integrated with the European Union and eventially NATO the map totally changes. Have a good look at the map and imagine Ukraine not being part of or associated with Russia. Bielarus would be more or less surrounded by western democracies and it would only be a matter of time untill it would feel that influence and at least would start to become more democratic and pluralistic and market orientated.
Moldavia and transdistrn would be much more difficult to hold on to if Ukraine became totally European and eventually associated with NATO. The rogue public would be a Russiancontrolled island surrounded by NATO or westernized armies.
and if you look at the black sea - at a moment in which Russia is investing billions in warships and started a war over Crimea you see that the strategic advantage will change if the coastline of Ukraine falls into the hands of the prorussian militias and the russian army in Ukraine.
You also see why the ex Yougoslavian republics will become member of the European Union in the end
and this makes (except for Norwegen) the map more or less complete
and Moscow is not so far from the Ukranian border making the Russian first strike impossible or much too dangerous even if Russian generals have placed it on the table now
this is the big endgame the global strategy all the rest is day-to-day politics and news
moldavia will not join NATO for the moment but as Putin is doing very hard his best to scare everybody, this is the map of NATO members and candidates and those who are not a member yet