Despite Kevin Feige's long-held claims that the MCU has been planned since the very beginning, it's not all that difficult to find evidence that the films we actually got were sometimes wildly different from what they started out as.
For instance, Ant-Man would have been entirely different with Edgar Wright and there's no way either Kenneth Branagh or Patty Jenkins would have made Thor: The Dark World the same way Alan Taylor did. Even on a much smaller scale, there's lots of evidence showing whole characters who were supposed to appear in movies and entire scenes that simply didn't make it past the planning or editing stage.
In some cases, we get to see those scenes added to the blu-ray release to help peel back the curtain on the film-making journey, but other scenes don't actually get that far. Some exist only in conversations, some in early script drafts and some actually get to the concept art stage, but each would have changed the make-up of Marvel movies in some pretty big ways.
In short, the MCU could have looked extremely different...