The following contains spoilers for Avengers: Infinity War. If you still haven't seen it, there may be no helping you.
Most would agree that the ending of Avengers: Infinity War was absolutely insane. Many fans had trouble believing what we were seeing the first time we saw it. However, Kevin Smith believes he knows how it could have been better. During a recent video cast, Smith talked about a conversation he had with his friend, Comic Book Men's Walter Flannigan, who thought Avengers: Infinity War was a great movie, but thought the film's post-credits sequence was going to go in a very different direction. According to Smith...
The final scene of Avengers: Infinity War shows Nick Fury sending a message on a pager. The camera slowly zooms in on it to reveal the logo of one Captain Marvel, a character we will meet in the Marvel Cinematic Universe next year. However, it would have ended a very different film, as Kevin Smith says, if the logo we had seen had been a number four.
Seeing a reference to the Fantastic Four at the end of Avengers: Infinity War would have been an epic moment for more than one reason. It would have been literally the last thing anybody in the theater would have expected to see. The film rights for those particular characters still don't belong to Marvel, and while the recent business dealings between Disney and Fox could eventually mean the Fantastic Four will find their way into the MCU, nobody expects to see anything like that happen in the near future.
If something had been happening behind the scenes that had been kept quiet all this time, Marvel fans in the theater would have come unglued if the post-credits scene had revealed it. Of course, that would also mean that the Fantastic Four were made part of Avengers 4 without anybody knowing, and that sounds pretty close to impossible, or at least like it would have been a major headache to work out, even if the deal between Disney and Fox had gone through already.
Of course, this idea does make one realize that when and if the Fantastic Four actually do make their appearance in the Marvel Cinematic Universe, it could be big. While the characters have had something of a rough go of it on the big screen, perhaps joining the MCU is exactly what they need.
Instead, we'll see Captain Marvel try to save the day in Avengers 4. What the future holds for the Fantastic Four, only time will tell.