I mean I guess the original thought was Mordo to be the villain in the sequel since the end of the first movie teased him of being the next villain

However I guess both Scott Derrickson and Sam Raimi wanted a different villain that being Scarlet Witch

They never said he would be the villain of that movie. Just a villain for a future movie. A Doctor Strange sequel was only the most likely, but not guaranteed. And of course, plans change.

Of course, there’s always another movie to do that.