In OTL, there is 28 member states of the European Union, with Croatia being the newest member having joined on the First of July, 2013. But, in the past, Norway was trying to apply but it didn't pass because they rejected, and Greenland left it after they had more home-rule. And also, the other (mostly eastern) European nations either showed no interest of ever joining, or they are more concentrated on Russia, or they have a long way to go to improve their country.
Though, if there was more dislike for Russia by the Eastern European nations, and they improved their country better? What if, their disputes of borders and (for Serbia, Georgia, Azerbaijan, and Armenia) loose nations (like Nagorno-Karabakh) were resolved? And, they would be capable of joining earlier? Also what would happen of Norway became a part of the EU and Greenland stayed? What is the fate of Switzerland? How about the small-city states that are in Europe (like Monaco and San Marino)? How will Russia react to all this?
(This is a open time-line. Feel free to change anything that would seem wrong on the time-line)
21 July 1967: Norway accepts the referendum, and joins.
1985: Greenland votes to stay.
1996: Ukraine sends in a referendum/application to join?