If Fox decides to cut it's losses and give back or (more likely) do a Sony type deal. To avoid doing a full orgin story the MCU is uniquely equipped to introduce these characters over time in bits and pieces.
First things first, the Marvel one shot. The one shot could set up a lot of the hows and whys of the story that the Fox movies lack. I would add a one shot of the aftermath of the Sokovia incident in Age of Ultron. In a cable news story where the whole country has been annexed by their neighboring country of Latveria and the destroyed Ultronbots are being collected by Latverian soldiers. Add in a press conference by the President of Latveria Victor Von Doom on the anger felt by the region from the actions of the international vigilantes the Avengers.
Another plus to the Marvel shared universe is the mid and post credit scenes. Add a random flashback to show how a janitor became father to the smartest man on the planet by stealing an "intelligence booster" formula from a stark lab. In it he sees the plans and formula and puts themin his pocket then a close up on his name tag that says richards.
I still think the whole getting their powers part should still be in the movie , but you have to add context the ff works best as part of a larger world.
Now for the cast. I don't like the whole young fantastic 4 mostly because as fox has proved it doesn't feel like the 4 we've grown up with. And Dr.Doom shouldn't be an old friend turned foe because it will always feel like a retcon. And speaking of Doom, he is one of my favorite comic characters so to see him in the Fox movies has been painful.
The actor I would like to see as the eastern European dictator is Marton Csokas from xxx, the Lord of the ring movies, the equalizer, the Bourne supremacy, ect. He could bring the right tone and feel of a Putin-like, Tony Starkish (minus the puns) Dr.Doom who's country was effected by Nazi and USSR incursion and made him like Hitler without the genocide and antisemitism.
Mr. Fantastic should be a strong likeable yet awkward leader. Personality wise, he should be a less comedic Sheldon from "the big bang theory". A male Dr.Brennan from "Bones" and some "Castle" thrown in, so naturally Nathan Fillion would be perfect.
The fantastic 4 should be a family so, when it comes to Sue Storm she should be motherly, highly intelligent, yet also beautiful enough to attract an eastern European dictator and an under water king. For her it has to be Ali Larter she encompasses everything sue needs to be.
The Human torch should be a young, good looking,hot head. But he also has to fit in the family unit. He's more of a little brother to Ben and a son to Sue. I know he's not been exceptionally good in his movies but Hayden Christensen could work well as Johnny Storm.
Now the final piece of the puzzle, the Thing . This one could go one of two ways. You could add diversity here because for most of the movie he will be a cgi rock monster. So you could go with Lester Speight from "Transformers : dark of the moon" he could add a comedic, entertaining and likeable likeable Thing to the family dynamic. He's a loud, boisterous, football player who. Could deliver the line "it's clobbering time" and it not seem forced or unnatural. That would be my choice. But,if you're more of a comic purest and want the ever loving blue eyed Thing definitely Kevin Durand. He wouldn't be as fun as Speight but he could pull it off.