Are these major characters about to hit the public domain?
Marvel is suing to make sure they never lose hold of some of their most important characters like Iron Man, Spider-Man, Dr. Strange, Ant-Man, Hawkeye, Black Widow, Falcon, Thor, and others.
Basically, these characters were created outside of Disney/Marvel by people like Stan Lee, Steve Ditko, and Gene Colan. That means that after their death, their heirs can ask to terminate the copyright, therefore taking the characters and reverting the rights to the heirs, and not the people who own them.
This all falls under the Copyright Act of 1976, which allows the heirs to cancel licenses granted or transferred for a copyrighted work in particular situations with a fully executed notice. Marvel believes it cannot let that happen.
Take the creation of Spider-Man as an example. He was created by Steve Ditko in 1962. Ditko's heirs have filed to terminate the copyright. According to The Hollywood Reporter, "Under the termination provisions of copyright law, authors or their heirs can reclaim rights once granted to publishers after waiting a statutory set period of time. According to the termination notice, Marvel would have to give up Ditko’s rights to its iconic character in June 2023."
Marvel has been handling many of these cases over the years. It was bound to happen when these movies began to make billions of dollars. Of course, DC has faced all these similar lawsuits and won, so there's no real worry as of right now. But all of these iconic characters seem to be up for copyright termination, so it'll be interesting to see where these land.
Marvel's lead counsel said, “Any contributions Steve Ditko made to the Works were done at Marvel’s expense because Marvel paid Steve Ditko a per-page rate for his contributions, Steve Ditko made those contributions to the Works with the expectation that Marvel would pay him, and Steve Ditko did not obtain any ownership interest in or to his contributions.”
The keyword here is that Marvel is trying to keep complete ownership. Some of the courts may determine that Marvel has to share the rights of the characters. That's all up to speculation now.
You can read their formal complaints here.