It seems like a movie revolving around Brian Michael Bendis' Dark Avengers could be a part of the post-Avengers 4 Marvel Cinematic Universe.
The untitled fourth Avengers installment is going to be the final installment of MCU Phase Three. We know about a few projects that are coming next - Spider-Man: Far From Home, Guardians of the Galaxy Vol. 3 (currently on hold), a Black Widow standalone movie and one based on the Eternals, with a focus on Sersi. But that's the extent of what we know about the future of our beloved Marvel Comics-based shared cinematic universe.
However, a rumor has surfaced on web that may have revealed another project that could be a part of the MCU beyond Avengers 4. That Hashtag Show is reporting that a script for a Dark Avengers movie has been developed and completed by Marvel Studios. But that's literally all that the website has learned - they cannot offer any further information on the project.
As cautioned by THS in their report, Marvel keeps developing scripts for a variety of different potential projects. Some of these scripts see the light of day and a bunch others simply don't. For instance, the studio once had Black Panther co-writer Joe Robert Cole working on a War Machine spinoff - a project that was later abandoned, simply to not contradict with the events of Iron Man 3.
So, it would be unwise to be already convinced that a Dark Avengers movie is happening. Also, Kevin Feige is on the verge of getting his hands on the X-Men and all Marvel Comics characters that currently belong to 20th Century Fox. When that happens, there would be a big change of direction, far bigger than the usual ones.
