According to new job listings, Blizzard is recruiting two lead software engineers to work on "a robust first-person engine for an unannounced project". It's not especially interesting that Blizzard is recruiting new engineers, but it is very interesting that it's for an unannounced project in a first-person engine.
As most will be aware of, Overwatch is the only game Blizzard has developed in a first-person engine, and that engine was built internally. While it feels kinda unlikely that the studio is working on another first-person game distinct from Overwatch, my instinct tells me it could be for a major expansion or, perhaps (gasp), a campaign component for that game.
The listings don't really hold any further clues. You could assume, I guess, that "collaborate with the art and design teams to achieve and extend the overall vision of the game" could mean that the successful applicant will be "extending" an already established IP. That's blind speculation though, and I like a few beers before I blindly speculate.