Any further EU expansion in an easterly direction is going to upset Mr Putin, although he has possibly given up on Serbia, Montenegro, Albania, Bosnia Herzegovina and Macedonia for the time being. Some efforts will be exerted to maintain the traditional Russian influance in Serbia but nothing compared to the efforts that will be exerted in the areas of Moldova, Georgia and the other countries which are closer to Russia and which were part of the Soviet Union. The three Baltic states of Estonia, Latvia and Lituania could see significant efforts at destabilisation coming from the very significant Russian populations within these countries.
I think that the EU needs to have a long hard look at the NATO arrangement and seriously consider an EU Defence arrangement, as suggested by Herodotus, which will operate outside of the NATO structure as the NATO structure is very heavily influanced by American Foreign Policy. Such a Defence Structure would allow the EU the flexibility to operate as a single military and foreign policy entity and take whatever action that the EU considers most appropriate in these circumstances and not be reliant on the US to rubber stamp what ever action the EU considers is necessary.