Sam Rockwell appeared as Justin Hammer in Jon Favreau's Iron Man 2, which released in 2010. While Mickey Rourke's Ivan Vanko/Whiplash died before the credits rolled, Hammer was arrested and jailed for his crimes, he later appeared in 2014's All Hail the King one-shot, which was the last time fans saw him. Rockwell has now revealed he wants to return to the Marvel Cinematic Universe.
During a recent Reddit AMA, Rockwell was asked whether he'd like to play Hammer again. He responded, "Yeah, I would be happy to do it! I'm just waiting on the avengers [sic] have to give me a call."
While Hammer was mentioned in passing during a single episode of Luke Cage, Rockwell hasn't appeared in the MCU for a while now. As it stands, Hammer is presumably still incarcerated at Seagate Prison, so any appearances going forward would probably necessitate him breaking out.
It's unclear where the Iron Man arc goes after Avengers: Endgame and and if there are any plans for Iron Man 4. After the divisive response to Shane Black's Iron Man 3, the de facto lead of the MCU has yet to return for another standalone movie.
If Rockwell did reprise his role as the CEO of Hammer Industries, it wouldn't be the first time a forgotten villain has made their MCU return. As well as Tom Hiddleston's many appearances as Loki, Lee Pace recently returned as Ronan the Accuser for Captain Marvel and Red Skull made a surprise cameo in Avengers: Infinity War.