Second Tier (by 2020, I'd be surprised if the dominos hadn't fallen)
Greece, Italy, N. Ireland, Austria, Germany, Nepal, Malta, Costa Rica, Estonia
I doubt whether Northern Ireland or Estonia will have marriage equality by 2020 - the latter pretty much needs the DUP to change their mind on the thing at least to the point of allowing a majority vote and that's rather unlikely; and although Estonia is one of the better former Eastern Block countries on LGBT rights (definitely behind Croatia, probably also behind the Czech Republic) I don't think that public opinion will have shifted on the issue enough to legalise marriage equality - the Russian population is still very hostile to registered unions, I'd imagine that support for marriage is even lower. I can't see the Centre Party supporting the thing - if they did then it'd definitely hurt them amongst the Russian minority and that's a huge part of their vote - and I don't think that there'd be enough support among the "Estonian" parties to get the thing through. I'm pretty sure that Estonia will legislate it before the other two Baltic states though; Latvia has a much larger Russian population and the Latvian population seems to support LGBT rights less than Estonia (although the most recent polling I could find was a Eurobarometer from 2006); while Lithuania has a terrible record on LGBT rights (they are the only EU country to have a Russian-style "propaganda" law) and support for civil partnerships is sub-10% in the most recent polls that I could find. I think that the latter is a huge shame: that's admittedly because I've been to the place (uni exchange in Klaipeda: really nice city that I'd recommend going to if you're near, nice beaches which surprised me) and thought that it was an incredibly nice place that I'd go to again, and it'd be only made better if they'd make positive steps forward in this area.
Everywhere else on that list seems plausible - I'm pretty sure that it passed majority support in Germany a fair while ago and its just the CDU/CSU holding the thing up, Italy will get it if Renzi gets a good result in the next election, and the others I can't say anything concrete about because I don't know much about them. If it was not for the referendum a few years ago I think that you could have put Croatia on that list just based on the good stuff that they've done recently; but I think that its very unlikely there for a fair while because they managed to get a ban in the constitution.
My understanding is that the issue is more religious - ethnic Estonians are mostly Lutheran (or of Lutheran background; most Estonians are non-religious), and the various Lutheran churches in Europe don't actively oppose LGBT rights, but ethnic Russians are Russian Orthodox, which church is vocally and vociferously anti-LGBT (and ethnic Russians are also much more likely to identify as religious). This is also the case in Latvia, though ethnic Latvians are about equally divided between Lutherans and Catholics rather than mostly Lutheran so less LGBT-friendly than ethnic Estonians. Lithuania is mostly Catholic and thus less LGBT-friendly than Latvia.
