I would love him as Mr. Fantastic if Marvel got FF back. Though not with Affleck as the director- that's kinda outside Ben's wheelhouse.
he'd be amazing as the question. it's just the sort of unconventional, yet action-packed role that damon would likely excel at.
Hawkman - we've seen two low-rent versions on tv in the last decade, time for the movies to get it right. And Damon can work without a shirt, no problem.
Adam Strange - Bourne in space.
The Question would be great, but how many actors want to work without their face? (it's a good thing RDJ always gets to have his close-ups inside the helmet)
Orion
Bring in the multiverse and make him Owlman. Matt as Owlman vs Ben's Batman would be an awesome set up for a big JL vs Crime Syndicate movie.
He would be a great Green Arrow. They could even do a little wink to the comic book fans who know that GA was kind of a copy of Batman. Have an antagonistic relationship with Ollie and Ben's Batman. over the years Ollie has become one of the characters who doesn't fear the Bat's spooky routine and will tell him how it is right to his face. Would be cool to see Ben and Matt recreate that on screen.