I dont have any confirmation for this, but I SWEAR that Wanda was supposed to be in MoM since they named it that, which was like 5 years ago or something.
I very much remember feeling like it would be the perfect time for her to be a villain and I know I read an article somewhere suggesting the same thing.
The problem with MoM is that there was never a clear vision for it. Benedict Cumberbatch and Elizabeth Olsen made jokes that the only thing about the movie that never changed was the name.
I'm sure there were plenty of versions that were great (or could have been) but it didn't fit in with the larger narrative of phase 4 so they kept changing it.