Even though part of the MCU's success has always been aportioned to how well-planned the whole expanded universe was from the moment it occurred to Kevin Feige, there's enough information out there to suggest that lots of the movies came extremely close to being completely different.
There's no way Edgar Wright's Ant-Man was anything like Peyton Reed's for instance, or Jon Favreau's Iron Man 3, or Patty Jenkins' Thor 2... And Joss Whedon was always pretty clear on how different Age Of Ultron was to his intial plans. That's just how films evolve over time, and no amount of forward planning would change that.
Even on a smaller level, we've seen that the MCU almost saw some wildly different specific scenes, and even further, it seems that some of the characters who ended up in the first ten years of Marvel's tentpole franchise could have looked radically different. Sometimes that's for the better, but sometimes there are what ifs that may have even looked cooler.
Thanks to the raft of concept art that's out there now, we have a fairly good insight into what the Avengers COULD have looked like had their alternate looks been approved...