A Hearts of Iron IV mod that allows for the integration of other European states into the EU once formed. This even includes European countries that are part of the USSR, as well as Turkey. No new countries can form the EU however, only democratic Germany, Italy, France, Netherlands, Belgium and Luxembourg, as well as the UK once the Imperial Federation focus is completed and Oswald Mosley is the country leader.
In the case of integrating states that used to belong to Yugoslavia or the USSR, the decisions to integrate these will also cause Yugoslavia or the USSR to lose cores on these states. This can come in handy when making puppets or collaboration governments from these countries.