When the latest iteration of Marvel's release slate became public knowledge, Walt Disney Studios revealed to fans that there is an as-yet-unannounced movie set to hit theaters in 2022. The film, an untitled Marvel project, will be released in between Captain Marvel 2 and Black Panther 2. The movie is sandwiched between a pair of sequels, but might actually turn out to be a new property -- or at least new to Disney. There are three movies that seem the most likely candidates to slot in (assuming there isn't a surprise in development, but that seems unlikely at this point).
In Disney's latest release calendar, the October 7, 2022 release date is still open as an untitled Marvel movie, and a number of films we already know to be in the works haven't yet got release dates. The film that seems most likely to be an October 2022 release is probably Blade, which has filmmakers and a star attached.
The film would also likely benefit from an October release, closer to Halloween, since it's a horror-themed superhero flick.
The other options are a little less clear. Ant-Man & the Wasp: Quantumania seems like a solid choice, for many of the same reasons. Granted, it (like Blade) isn't yet in production, but all of the pieces are in place and if the screenplay comes together quickly they easily could.
Somewhat less likely, but certainly not outside of the realm of probability, is Deadpool 3. That franchise came to Disney from their Fox acquisition, and there's no screenplay or anything yet, but that doesn't mean a project with so much already established in terms of its look, feel, the costumes, the cast, etc., couldn't easily get off the ground.
There's also stuff like Fantastic Four and other projects that have been announced, but are unlikely to be even close to ready by mid-2022. Another thing to consider is that Black Panther II might need some extra time to get prepped, considering that franchise star Chadwick Boseman passed away. That means any of these movies we're talking about could theoretically be slotted in to replace that film if it were to get a delay of its own.