Iron Man 4 has always been a wild card with Marvel and the Marvel Cinematic Universe. Obviously, audiences love Robert Downey Jr. and the Iron Man series- it's because of that that the most recent entry in the series, Iron Man 3, grossed over a billion dollars, making it one of the most successful superhero movies of all time.

But with Iron Man 3, Marvel also seems to be done with solo Iron Man movies, leaving the character to appear in Avengers movies and the upcoming Captain America: Civil War, and nothing else. You'd think the studio might realize what a cash cow Iron Man is and continue to put him in more sequels, but it doesn't seem to be going that way.

Although there is hope for Iron Man 4- some rumors have the film premiering on May 6, 2016, while other rumors peg the film for a release date in 2019 or beyond. And when the next Iron Man movie does premiere, who will the villain be? Because every superhero needs a classic villain to fight against its legendary hero.

There are a few potential options. It's been rumored that Jeff Bridges' character from the first film, Obadiah Stane (aka the Iron Monger) may not actually be dead and may appear in a future Iron Man film. As well, there's always the Mandarin- rumored to be Ben Kingsley in Iron Man 3, then revealed to actually be played by Guy Pearce. And then, in a later short film, it was revealed that both Mandarins were hoaxes, meaning the real Mandarin is out there somewhere- potentially plotting against Tony Stark.

Newsarama puts those two characters- Iron Monger and Mandarin- as Iron Man's greatest villains, so they're the most likely to show up in a sequel somewhere. Most likely, Iron Man 4 (or maybe Iron Man 5?) whenever Marvel finally gets around to making one.