How are the EU candidate countries progressing?
Well, from my understanding Montenegro is the only realistic EU candidate for the 2020s. Talks in Montenegro seem to be advancing at a very slow but steady rate. I can't think of any possible issues with Montenegrin accession beyond the talks themselves, so I think Montenegro will make it in by the end of the decade.
Serbia is the only other candidate with chapters open and closed. It's advancing even slower than Montenegro, but not too much slower. In fact I imagine the EU probably wants some sort of joint Serbia+Montenegro accession. However the main issue with Serbia is the fact that it has severe democratic deficits plus as you may expect the issue of Kosovo. I do not expect these issues to be solved any time soon so I don't expect Serbia to join the EU at all.
Beyond that Albania and North Macedonia have just started negotiations but no chapters are open. Still at the current rate they'll eventually make it into the EU and the only issue they have (the North Macedonia naming issue with Greece) is solved. However they won't make it until at least the mid 2030s.
Bosnia is the final European state that is even remotely realistic for accession, but given its issues it almost certainly won't make into the EU until at the very least the mid-late 2040s, if ever.
Kosovo will never make it until it solves its issues with Serbia. Neither will Belarus or Russia.
Ukraine will never be let into the EU for similar reasons as to why it probably won't ever join NATO. However, in a vacuum if Russia ever gets reasonable, it is better than Bosnia as a candidate; I'd put it on the Albania/North Macedonia group
Worth noting a hypothetical independent Scotland or Catalonia could make it into the EU with relative ease but of course I don't expect either of the 2 to happen. Similarly, if Norway, Switzerland or Iceland ever apply to join, it is a guarantee that they will make it in in around 5 years or less; it's just very unlikely that they ever will apply.
Anyways to sum it up:
Montenegro: Late 2020s
Serbia: Early 2030s, but only if it gets the Kosovo issue sorted out
Albania+North Macedonia: Mid-late 2030s
Bosnia: Beyond 2040
Independent Catalonia/Scotland: 5-10 years from independence, assuming Spain does not veto
Norway/Iceland/Switzerland: 5 years from application date
Scotland would make it. Also if Northern Ireland votes to join Ireland (Catholics have higher birth rate than Protestants so will eventually be a majority) it automatically becomes an EU member much like East Germany did when re-uniting. Same with Northern Cyprus, automatically a member if island re-unites.
Catalonia I don't think ever joins as an independent one as for starters Spain won't let it separate and if it were to separate, Spain would veto its entry.
Turkey: Not likely before 2040. As long as Erdogan or an authoritarian like figure is in charge won't even be considered and even after that its almost a guarantee at least one country would veto their entry thus not happening anytime soon.
Final one forgotten is the United Kingdom.
I don't think it joins anytime soon even if they want to re-join but I do think with younger Brits mostly being pro-EU it does eventually re-join but probably not until around 2050 or so. EU likely will require 2/3 majority in favour of re-joining so that know its secure and won't face another Brexit.
Other is Gibraltar choses to join Spain, but I see this as unlikely since despite Gibraltar voting 96% to remain in EU, I believe very few wish to join Spain even though that would automatically bring them back into the EU.
