The MCU has had success with cinema and both traditional and non-traditional television. So where to next?
How about animation? Films filling blanks, or about characters who will never have a solo movie or television series. Animated films, I'd imagine direct to video/streaming, of high quality, that fit into the established continuity. Films that are given appropriate care, with voice cast (if possible) the same as live-action counterparts.
Marvel as a whole, in my opinion, do not have a great record for animated films. DC have a better record, but with a few notable exceptions, also struggle to reach consistent quality.
My suggestions for a film slate:
Thor: Tales of Asgard
The mythology behind Thor is dense - both with the 50+ years of Marvel continuity and the actual Norse legends. They are also expensive films to make and I don't think it is a coincidence that both Thor films are amongst the shortest in run times (if not the shortest) for the MCU.
Animation seems a perfect fit. Under used characters (Warriors 3), or characters that haven't appeared at all (Beta Ray Bill/Balder/Enchantress), could be given starring roles. You could introduce more alien species, go to more realms, and it wouldn't have to impact the ongoing MCU universe.
Nick Fury: Agent of S.H.I.E.L.D.
A solo film is not going to happen. A TV series? Maybe. But I'd love a Nick Fury prequel story (animation would allow him to voice the role without being recast). We could meet his Howling Commandos, see younger versions of Alexander Pierce, Peggy Carter, Howard Stark and Edwin Jarvis - or even get some proper Hank Pym and Janet van Dyne interactions.
Many, many, many options. The character is very popular.
Black Widow: Origin
Yes, she has had 4 films and counting, but her origin only hinted at. A noir/spy animated film, with supporting roles from characters such as Hawkeye, Phil Coulson and the Winter Soldier.
Howard the Duck
Why not? He already exists. Yeah. You'd watch it, you know you would. A journey into the stranger and more cosmic side of the universe, or have him interacting in modern day Earth. Or both.
Fantastic Four
Yes, Marvel don't own them. But let's say the rights expire and are reverted back to the MCU. What are they meant to do? The general movie going audience have a bad taste with Marvel's first family, and they aren't likely to spend big money on this film when they could do another Guardians of the Galaxy instead.
It doesn't have to be an origin, but it can tell a good Fantastic Four story. And then the characters could be free to appear in the movies going forward.
The Incredible Hulk 2
Marvel have left some big story threads hanging since 2008's The Incredible Hulk. Notably the Leader, and the Abomination. You could set it in a time between Avengers: Age of Ultron and Thor: Ragnarok, and use it as a prequel to what ever happens in Thor 3 and tie up loose ends at the same time.
What films would you like to see? Which characters do you not think will get a proper live-action adaptation, that you'd like to see animated instead?
Now will these films make money? That's the big question, and a question I can't answer. I'd pay for them!