At this point, it seems like every Marvel character has shown up in the MCU but there's still one character that Stan Lee would love to see added to the universe.
Talking to Syfy.com, Lee revealed that he would love to see the Silver Surfer at the centre of an MCU movie. The character featured in the 2007 film Fantastic Four: Rise of the Silver Surfer which received heavy criticism from film goers and critics alike. Lee hopes another filmmaker may get another chance to do the character justice saying "He’s the one I want to see more,” Lee said. “He’s the most philosophical of all the characters I’ve ever worked on. And I have an idea for my cameo in that one!"
Unfortunately for Lee, it might be a while before that happens. 20th Century Fox still hold the rights to the Fantastic Four property, and probably wouldn't be so eager to hand control to Marvel and Disney. That being said, Marvel and Sony did work out a deal for Spider-Man, so it might be possible.