According to the website of multimedia company StoryLab Productions, they are working with Gears of War 4 developer The Coalition on a brand new IP - seemingly unconnected from the Gears of War franchise.
StoryLab Productions, a multimedia production company with close ties to Microsoft who worked on both Gears of War 4 and Gears of War: Ultimate Edition, posted about their next project with The Coalition. There's not much information, but they say it's a "new IP" that they had begun last year. While Microsoft, in particular head of marketing Aaron Greenberg, has previously suggested that The Coalition would be dedicated to Gears of War, it's entirely possible this position may have changed and the studio could be working on a new action franchise alongside Gears 5.
This was first spotted by Spanish site Generacion Xbox, who recently posted a rumour that this new game at The Coalition would actually be a new game in the Perfect Dark franchise. Take that with a grain of salt, however, since it came from an anonymous forum poster.
If you don't know who StoryLab is or why they'd know about this "new IP", here is their introduction:
"As an official vendor to Microsoft’s ‘The Coalition’, StoryLab has supplied a wide range of services to this gaming giant including; Motion Capture Services, Casting Services, Stunts and Performers, Production Crew, Payroll Services, Production Consultation, etc."
While they don't make games specifically they help put the cutscenes and realistic animation together, and note that they are very proud about working with The Coalition in particular. They know all about Gears, and that the new game coming from them is not a Gears game. Very interesting.
At the very least, this'll bring the number of games in development by Microsoft to at least 3 - assuming either Sea of Thieves or Crackdown 3 don't get cancelled.