Croatia could only really be beaten in by Iceland, barring very unusual circumstances.
(Icelanders could well decide though that they don't want in, and may also face the British and/or Dutch stalling accession.)
An oddball pair of possibilities for future expansion would be Mauritius and Seychelles. Not as strange as one might think since neighboring Reunion is in the EU (as part of France) and Seychelles is arguably more developed than some of the current EU members.
Probably not the next wave of expansion unless that takes a long time to happen.
Being a European State is a sine qua non for membership.
Neither Mauritius nor the Seychelles fit the bill.