Look, I know that this is probably the millionth post about a Fox property, but please hear me out. I'll make it quick. Everything regarding the Fantastic Four from here on out is hypothetical. Let us begin!
Phase III Line Up
Now I might get cut for saying this, but say Marvel gets rights back (With or without X-Men). I would totally be okay with both Infinity War films being moved a year later. I firmly believe that the third Avengers film needs to be the end of Phase III. I feel like Marvel would do the same thing the did for Spider-Man: Homecoming. Remember that that film wasn't in the original line-up, but they squeezed it in. It probably won't happen, but I think they could squeeze it in before the final Avengers film. Remember, I'm giving a quick rundown. Nothing in depth, just really quick.
Who Will Play Them?
I think the old cast would be fine assuming any of them want to do it again and find someone else to play the Human Torch because Chris Evans is Captain America. If Marvel Studios for some reason does the race change, mess with Ben Grimm and not Johnny Storm.
How Would They Fit In?
So where were the Fantastic Four during the Battle of New York? Heroes have their own problems to deal with. The Fantastic Four during this film would have been in the Negative Zone dealing with a threat. They come back and find out what's happened.
Where were the Fantastic Four here? Simple. Say they helped Fury get the Helicarrier up and running, but had to go to Latveria to tango with Doom so they didn't have time to assist in Sokovia.
For Civil War, actually I have a simpler answer. Since the Sokovia Accords are simpler than the Superhuman Registration Act, my answer will surprise you. The Avengers are the mightiest heroes so you know when they're in town. The Fantastic Four don't cause as much destruction and they'll be off in other dimensions most of the time. The X-Men didn't get involved with the Avengers in the comics during this conflict. I think that the Sokovia Accords would be confined to the Avengers only. However, Reed Richards gives Tony Stark some advice and then he heads off to deal with Super Skrull or somebody.
How Do They Know Each Other?
Even though this is mainly Avengers universe characters, you can see the point. Everyone is united and together the way they should be in the MCU. S.H.I.E.L.D. is how they would know each other.
Nick Fury was forming the Avengers. Say that he approached Reed Richards and he refused. Say he approaches Wolverine. Either way, he'll have S.H.I.E.L.D. files on them and the Avengers can read up from there. Also, I feel like Tony Stark and Reed Richards would already know each other. I also feel like Reed would also know Jane Foster and Eric Selvig.
Who Does The F4 Bring to the Table?
This is probably the most important point right here. Their villains are often fighting the Avengers. I'm only doing the villains that the Avengers deal with directly.
1. Dr. Doom
I chose this pic from the cartoon Avengers Assemble to prove the point of F4 villains fighting the Avengers. Dr. Doom is a much needed villain in the MCU if you ask me. Personally, I think Benedict Cumberbatch, who plays Doctor Strange, would have made a better Doom.
2. Silver Surfer
Most of the Fantastic Four's villains are cosmic so I'd really like to see the Guardians of the Galaxy take on Silver Surfer before the Avengers get to him.
3. The Skrulls
Thankfully, Marvel has the rights to the Kree. Even though the Kree were focused on the Zandarians in the Guardians of the Galaxy film, there might be room for the Skrulls and we could get our Kree-Skrull War.
4. Kang the Conqueror
Kang is a time traveler and I think it would be fun to see that aspect be brought to the MCU.
Galactus has been cause for many a superhero team up. If Marvel gets the Fantastic Four, Galactus is definitely coming for the Avengers.
And that's it for today!