The original ending to Freddy vs. Jason was supposed to have Pinhead chain the two, and say "What is the problem?" But some people in charge felt Pinhead was not popular enough, so they decided not too. So that is one pretty interesting possibility. There was also the idea of bringing both Myers and Jamie Lee Curtis into the mix, one would imagine it would become confusing for characters to know if Jason or Myers were doing the killings. This is of course before RobZ took over for Halloween, so god knows what Freddy might have turned into a nightmare for Meyers (before we were shown what was on his mind in Halloween II).
There was also the idea of "Helloween," a Pinhead vs. Meyers movie. More people screwed this up.
So instead, we got stuck with the awful Friday The 13th remake, an ok Nightmare On Elm Street remake, and pretty cool (2) Halloween movies.