There should be something like an associate member status and a probationary period, This basically is what happened in the US. A territory, once it had its shit sorted and kept it so, could apply and would be admitted as a state (or sometimes several).
Contrast this with Hungary's admission:
EU: Your economy is too state controlled and your political system is so corrupt it makes the Eyeties blush. Sort it, and you can come in.
Hu. We promise we will, really. Let us in!
EU: About those reforms
Hu: Nerny nerny ner ner!