Originally Posted by JJJ's Ulcer
In modern superhero movies, there are very few films in which the villains return. I can think of Magneto and his Brotherhood, Loki and Scarecrow.
I agree with this. They can't sell new toys if they keep using the same villains over and over. Only Loki and Magneto come to mind as villains that have had significant roles in the sequels. (That was not Dr Doom but an impostor).
Let's say they Mandarin was Mandarin all along. I don't see how they could have used him for future unless they went with the "teamup with the hero" route which has been done already.