Um, dude, there's only one villain who can unite them all and in the darkness bind them....
MORBIUS!!!
Ol' Webhead has even managed to fight alongside the Punisher. If comics can do it, so can film.
I could see Spidey/GR, but ideally my team-up movie for Spider-Man would be if the rights go back to Marvel, have a grittier, street-level crime drama with Spidey, Daredevil, and Punisher, with Spider-Man and Daredevil reluctantly having to team up with Punisher to take down Kingpin or something. While the overall tone and story would be dark, I would say definitely keep Spider-Man as "light" as possible to contrast, and show the severity of the situation.
Very Cool Pics Spidey Vs Ghost Rider
I have been thinking that for some time now. The strange thing is that some people argument against it...If The Avengers hit big the studios will be lumping all the characters they have together in team-up films.