Okay, so.
Here's what we know for sure about Marvel's release schedule:
05/01/15: Avengers: Age of Ultron
07/17/15: Ant-Man
05/06/16: Captain America 3
07/08/16: Doctor Strange
05/05/17: ???
07/28/17: Guardians of the Galaxy 2
11/03/17: ???
05/04/18: ???
07/07/18: ???
11/02/18: ???
05/03/19: ???
Now, I think the Civil War storyline could make this pretty interesting. Here's what PH's last guess on the schedule was, from 07/26:
05/01/15: Avengers: Age of Ultron
07/17/15: Ant-Man
05/06/16: Captain America 3
07/08/16: Doctor Strange
05/05/17: ??? (Iron Man 4)
07/28/17: Guardians of the Galaxy 2
11/03/17: ??? (Thor 3?)
05/04/18: ??? (Avengers 3)
07/07/18: ??? (Captain Marvel)
11/02/18: ???
05/03/19: ??? (Avengers 4)
I have a bit of a different take now, and it all starts with that 05/05/17 slot. I think Iron Man 4 is sort of unlikely to happen at this point (or, if it comes, it won't come so soon) (or, if it comes so soon, is more likely to be an ensemble Mighty Avengers/SHIELD movie in the aftermath of Civil War). I think that Doctor Strange could be used as backstory for him leading up to the underground New Avengers, and with the rumors that Cap 3 is being treated as "The Avengers 2.5," it would make sense to use Doctor Strange as a bridge from the Civil War to that New Avengers uprising in a possible Avengers 3. So, I'm thinking that Marvel could be going for the jugular by scheduling Avengers 3 that May, with GoTG 2 that August. Those two movies would absolutely slaughter the box office for Disney for about 6 months. (Edit: This contradicts the recent rumors that Marvel is planning on splitting A3 into two parts in May 2018-19.)
That's an aggressive line of thinking, though, and it's probably more likely that Marvel goes with a more conservative film in that slot. I think it's wide open... They could even introduce a new character like Nova/Richard Ryder just ahead of GoTG2. At the end of the day, I think the most likely film there is probably Captain Marvel. She's going to get a movie sooner or later, and she's rumored to be getting an introduction in Cap 3 (much like Dr Strange).
Here's what I'm thinking the schedule looks like:
05/01/15: Avengers: Age of Ultron
07/17/15: Ant-Man
05/06/16: Captain America: Civil War
07/08/16: Doctor Strange
05/05/17: Captain Marvel
07/28/17: Guardians of the Galaxy 2
11/03/17: Thor: Ragnarok
05/04/18: Black Panther
07/07/18: Avengers 3 Part 1 (Mighty vs New?)
11/02/18: ??? (Inhumans? Blade? Nick Fury / SHIELD? Nova?)
05/03/19: Avengers 3 Part 2 (Thanos?)
I'd still kill for Ph's idea of a standalone Loki film, too.