The Marvel universe has so many incredible characters running around that it is easy for some to just disappear. A few of the most powerful heroes have completely vanished from the regular storylines for years now. Fans are still waiting for their big return because their massive powers left a huge impression on everyone.
Sam Alexander Is Missing His Nova Helmet

Sam Alexander was a massive deal for a while as Nova especially when he was hanging out with the Champions. His special helmet gives him crazy energy powers and speed that put him on par with top tier heroes. He completely vanished after a big fight over his helmet and hasn’t had a solo story page since then.
Tamara Rahn Had to Leave Her Sea Family

Tamara Rahn is an alien with super strength and aquatic skills who fit right in with Namor and the Atlanteans. She was a fantastic underwater fighter but got hit with a special radiation that makes her dangerous to her own friends. Because of this tragic situation she vanished and hasn’t been back to the oceans since.
Quasar Left the Cosmic Stage Way Too Early

Wendell Vaughn used the incredible Quantum Bands to control cosmic energy on a level that few could ever match. He was the main guy for outer space adventures for a long time before his appearances started getting super rare. It has been quite a few years since he got a single panel in the main comics.
Thor Girl Ascended Straight Into the Stars

Tarene wanted to be the ultimate sidekick for the God of Thunder so she turned herself into an Asgardian goddess with limitless cosmic power. She spent a good decade fighting the good fight before re embracing her ancient role as the Designate. She flew off into the cosmos and left the superhero life behind for good.
Ultragirl Started Strong but Totally Faded Out

Tsu-Zana had her own solo comic series right from the beginning which is pretty rare for a brand new powerhouse hero. Everyone thought her super strength and energy manipulation would make her a permanent A-list character in the universe. Instead she got pushed into small supporting roles and has been gone for over ten years.
Blue Marvel Has Too Much Power for His Own Good

Adam Brashear is basically an unstoppable force who can lift a hundred tons and manipulate antimatter without breaking a sweat. He is easily one of the most powerful humans ever created but the writers just cannot seem to find a place for him. His epic team up ended without a sequel and he went missing.
Mar-Vell Left a Legacy That Nobody Wants to Touch

The original Captain Marvel was a legendary Kree warrior who protected the Earth with everything he had. He passed away from illness rather than a battle which gave his story a massive amount of emotional weight. Bringing him back would totally ruin that respect so he stays gone to honor his ultimate memory.
Slapstick Is Just Too Much of a Cartoon Joke

Steve Harmon got turned into unstable molecules which means he is an indestructible cartoon character who cannot even feel real pain. You would think a guy who can survive decapitation would be in every major fight but he is mostly a joke. Writers completely forgot about him after a short run alongside Deadpool.
Adam Warlock Simply Lowers the Stakes for Everyone

Adam Warlock is the perfect human with quantum magic, telepathy, and cosmic awareness that makes him way too overpowered. If he shows up to a fight the bad guys do not stand a chance so he stays away. Fans hope his movie appearance might bring him back to the pages eventually.
Nate Grey Completely Missed the Biggest Mutant Era

Nate Grey has reality shifting psionic abilities that literally make him a god walking among normal everyday humans. He is an ultra rare mutant powerhouse but he chose to exile himself into a completely different reality. He missed the whole massive Krakoa storyline and mutant fans are still missing him big time.