At Wizard World Philadelphia 2018, Michael Rosenbaum spoke at his panel and confirmed that he would be in a future Marvel movie. He could not confirm that it would be Guardians of the Galaxy Vol 3. “All James (Gunn) says is I’m in one of the next ones. I don’t know what that means. He won’t tell me exactly.”
Still, it’s likely he’ll be reprising his role as Martinex from Guardians of the Galaxy 2. Whether he’ll make an appearance in the next Avengers film as the battle continues against Thanos remains to be seen.
Responding to questions about his voice role as The Flash in Justice League Unlimited, he said “They still ask me to do Flash for… like… there’s another- there’s something coming out which I can’t say. It’s cool.”
Could he have been involved in the Death of Superman animated movie? (See the Cress Williams interview, who voices Steel in that project.) Or is this another project entirely? I guess we’ll have to wait and see.