Here is the updated timeline;
marvel1update october 8 2015.jpg
I know that Marvel Studios has 3 ways of working ;

2020 2 movies with established character(s) + 1 to be introduced
2020 1 movie with established character(s) + 2 to be introduced
2020 3 movies with established character(s)

http://www.slashfilm.com/marvel-movie-plans/
I find Guardians of the Galaxy 3 a very likely possibility.
Not so sure about Spider-Man 2 and Dr. Strange 2.

Thoughts?