If they do increase to 3 films a year perhaps we could get 7 films in phase 3, (ph1 was 6, ph2 just 5). Although 6 seems the most likely, I could see 7 films as ph3 if both 2016 and 2017 are 3 films each, plus Ant-man in 2015 equals 7. How I could see that/what I would like to see,
2015
Ant Man
2016
Cap 3
Doctor Strange
Thor 3
2017
GOTG2/and or Ms. Marvel
Avengers 3
Black Panther
for the 2017 films you could shuffle around Black Panther and the GOTG sequel which I think could be more of a Ms. Marvel spinoff/standalone film but still most closely/tied and set in the GOTG niche of the MCU, expanding that part of the universe/story. Either way which ever one comes first would depend on how and/or where they intend to take the story leading into and out of Avengers 3. That would be my ideal phase 3.
Although perhaps looking at it again, maybe what ever film is post Avengers 3, would possibly be phase 4 already. The only ones I can see as possibly certain right now for phase 4 might be Iron man 4 and Avengers 4 with possibly a Dr. Strange sequel. And of course hopefully we'll get an Inhumans film at this point. Possibly also a Hulk sequel perhaps and maybe some team-up flicks based on some of the traditional double-billings from the comics i.e Cap/Iron Man Tales of Suspense, maybe a Nick Fury/Dr. Strange Strange Tales, perhaps finally the Black Widow/Hawkeye Budapest prequel, maybe a Thor/Namor crossover? I'd like to see a Cap 4 too if possible although he could the Iron Man of phase 4 and return for phase 5. Hopefully the franchise will still be going strong.
Probably phase 4/5 is where I would expect to start seeing possible re-castings of the Major players or transitions in continuity/possible "reboots", although I'd like to see them take this version, at the very least continuity/timeline as far as they can and see how developed it can become.