Yeah, I got the impression they announce the film after they've signed the major players for it. Obviously that wasn't the case with their current movies (particularly Ant-Man), but it's what Kevin Feige said before Comic-Con. If they signed the people they needed to, they would announce. It appears they're still in negotiations.
Anyway, the May 2017 date is the biggest mystery for me. They keep saying they're trying to do one sequel one new per year. Since they already announced GotG2 and Dr. Strange is likely July 2016 (with Cap3 that year), it could very well be an opportunity for Black Panther or Captain Marvel or Inhumans. The other alternatives would be a Black Widow movie or a Hulk movie (technically a sequel, but, with a different actor, might feel like an original movie). I had expected Black Panther, Captain/Ms. Marvel, and the Inhumans to be Phase 4, but now I'm not so sure.
ETA: I'm really thinking May 2018 is Avengers 3. It seems set up so GotG2 and Thor 3 are the two prior films, which would seem to lead in nicely to Thanos.