The ones to watch for, imho, are Superman in 2034, Batman in 2035, and Captain America in 2036, since the barriers to entry for comics are so low compared to film. I know people in the comic industry are already talking about these.
I'm not holding my breath for Superman to enter the Marvel Universe and Captain America to enter the DC Universe.
Two of the oldest superheroes, the Scarlet Pimpernel and Zorro, have been in the public domain for years, but DC and Marvel have yet to include them. Many superheroes from the 30s and 40s are also public domain because no one bothered renewing their copyright. They've appeared recently in comics by Image and Dynamite, but again, DC and Marvel are not interested.
But the main reason you will probably never see Superman in Marvel is trademark law. When Mickey Mouse became public domain, Erik Larsen wasted no time putting him in his Savage Dragon comics. But even though he is called Mickey Mouse in the story, his name is never mentioned on the cover and the solicitations only refer to him as the Rascally Rodent. Larsen couldn't advertise using Mickey's name without violating Disney's trademark.
Marvel is even more careful. When they lost the rights to Fu Manchu, the father of Shang-Chi, they refused to say his name at all, even though Fu Manchu was in the public domain. They revealed that his real name was Zheng Zu and the other name was just an alias.
If they can't use Superman's name in marketing, why would Marvel bother with him? And I suspect that DC has also trademarked phrases like "Man of Steel" and "Man of Tomorrow".