10 Characters The MCU Wishes It Could've Introduced Differently
3. The Mandarin

In all honesty, I don't think Marvel really realised how much people cared when they made Iron Man 3. In the here and now we all know now that this wonderful interconnected shared universe gets scrutinised and poured over to within an inch of its life, with every single trivial detail fair game for theorising and extrapolation. But back in 2013, I think they thought they were just having some fun with some movies.
Thus when they announced that the villain for the closing chapter in Tony Stark's personal trilogy would be The Mandarin, historically his greatest ever adversary and a character who had given Marvel comics some of its greatest ever conflicts, fans were ecstatic. The Ben Kingsley showed up as... an actor... from England... called Trevor. You could hear the disappointed sighs from space.
After the inevitable (and justified) backlash over this, Marvel attempted to remedy it with an admittedly excellent One Shot sketch, but the damage had been done. Given the chance for a do-over, they'd either have rewritten Iron Man 3 entirely or kept The Mandarin on ice until they knew how to introduce him properly.