Not long ago there was a post that described a development that led to Angola - or at least large parts of it - to remain Portuguese.
One major problem is that the motherlands didn't invest sufficiently in the colonies, didn't want to invest any further, and didn't want to accept the former colonial subjects as full citizens. Otherwise substantial parts of the former colonies could have been kept.
---
I agree that islands are the most likely. I'd add to that "outpost" colonies, such as Spanish Sahara, Djibouti, Aden, the gulf emirates, Brunei, Goa, Guayana, Suriname, Belize, Mosquito, Spanish Morocco.