After such an epic, universe-spanning, all encompassing Superhero film there might be nowhere for Marvel to go... but they will of course want to continue to release films set in the MCU. To do this I think Marvel should take a very different approach after the release of Avengers 3 - instead of pushing forward with Phase 4 they should instead look to films set within the MCU that can 'slot in' to the then established timeline...
Below I outline which films/TV shows Marvel could explore;
Damage Control
Drew Pearce (co-writer of Iron Man 3, writer/director or Marvel One-Shot: All Hail The King) has been recently talking about a film based on these characters. If it was made after Avengers 3 it could be the MCU version of Ross/Busiek's 'Marvels' book. The story can take place over several years spanning Phase 1 - 3 with the team clearing up the aftermath of events in Iron Man 1 – 3, The Incredible Hulk, The Avengers, Thor 2 etc.
Black Widow & Hawkeye in Budapest
Fans have been intrigued by the backstory between these two characters, and in particular what happened in Budapest, since The Avengers. This could be an action/spy/thriller set sometime before The Avengers.
Runaways
A fairly self-contained story, based on the initial story arc in the comics.
Howling Commandos
Set around the first Captain America film. Show how the group got together, got captured and teamed up with Captain America. It can also flesh out the montage section in 'Captain America' when the team are destroying Red Skulls bases, and then following them after Cap 'dies'.
Young Nick Fury
We know Fury was involved with SHIELD early on, knew Howard Stark etc. We can finally find out how he lost his eye...
Young Hank Pym
It's unclear how much, if any, we will see of Pym's adventures as a young man in the upcoming Ant-Man film so this could be set entirely in the 1960's/70's with a tone to match.
Guardians of the Galaxy 2, or spin-offs
GotG will introduce lots of new characters, planets, species and concepts to the MCU. This opens up lot's of opportunities to do one or more films entirely away from earth.
Agent Carter
The TV show will be set after the first Captain America film and basically do it's own thing but throw in some fun little easter eggs relating to what we, the viewers, know happens later in the MCU.
Namor
Marvel Studios has the rights back to the character but are unlikely to find a way to fit him in before Avengers 3. This film can be set in Atlantis at pretty much any time in the timeline – maybe even during WW2 as per his original appearance...
Netflix, Defenders, Heroes for Hire etc
Assuming we see a Defenders team up in Avengers 3 we could then see a Defenders solo movie (perhaps bringing in Hulk, Dr Strange, Namor etc). A Heroes for Hire film or TV show could also be a good way to carry on with these characters, or simply more solo series on Netflix. If he hasn't already featured, they could bring in The Punisher too.
Blade
Bit of a long shot but could Marvel work Blade into the MCU? Maybe an action packed version of The X-Files with SHIELD investigating?
Planet Hulk/World War Hulk
Following Avengers 3 we could see Hulk banished from the Earth, setting up this storyline. If successful it could lead into a World War Hulk film – which would be a good way to bring The Avengers/Guardians of the Galaxy/The Defenders back together for a big crossover film...
Would you like to see Marvel pursue this idea, and what films would you like to see in the MCU after Avengers 3?