Marvel Comics have plenty of potential for sequels thanks to their movies basically printing money whether they're good or not. Their good guys need to go up against bad guys. And there are still some villains who are as bad as they come.
You're teasing us, Avengers. Marvel films are known for having a big reveal at the end, usually after the credits. A poorly kept secret was that Thanos, one of the strongest and most feared baddies in the Marvel universe, would make an appearance at the end of The Avengers. Yup, there he was, which led us to believe Thanos would be the antagonist of the sequel. Nope. Ultron will assume that role. We'll have to wait for a third Avengers. Thanos' major powers include superhuman strength, telekenisis, genius intellect and teleportation. He is more than a match for all of the Avengers.
A combination of Magneto and Professor X, Onslaught has made no appearance in any Marvel movie to date. Any film featuring him would be a raging actionfest. In the comics, it took a combination of the X-Men, Fantastic Four and Avengers to defeat the monster. That's some power. Onslaught has super strength and superior mental power, but most impirtantly, it can sense mutant presence, which would be a danger to any X-Men. Onslaught's appearance looks like a souped-up Magento costume. And Onslaught can take on any Marvel hero as it can increase its size and strength at any time.
Fantastic Four: Rise of the Silver Surfer totally messed this one up. Galactus never really made a true appearance. He was more of a cloud of smoke. Galactus is dangerous, like destroy-a-whole-world dangerous. Why would we be afraid of a bunch of smoke? Get it right, Fantastic Four. Galactus is the most god-like figure since it literally eats planets and is responsible for eliminating entire species. The deity is so incredible, its form is perceived according to what that race believes is their god, something that gives and takes life.
Another Marvel bad guy who deserved better. And wouldn't you know it, Fantastic Four ruined another villain. Dr. Doom is capable of taking on numerous heroes at the same time. Doom is a scientific mastermind, can control vasts amounts of energy, posseses power over machines, casts defensive force fields and shoots deadly electric bursts. He needs to get beefed up in any future Marvel films. Let's pretend the two Fantastic Four movies never happened.