To be clear, this is separate from all the jobs about Diablo 4.
Their jobs usually list which franchise the game is about (even for unannounced projects, like Hearthstone's had "Deep Knowledge of Warcraft") if it's in an existing IP, so it's plausibly in a new IP. They may have just decided to be less obvious though.
Job 1: http://us.blizzard.com/en-us/company...tml?id=160013G
Job 2: http://us.blizzard.com/en-us/company...tml?id=160013F
LEAD SOFTWARE ENGINEER, ENGINE
Blizzard Entertainment is seeking an experienced tools engineer to work on an unannounced project that utilizes a robust first-person engine. The ideal applicant has demonstrated ability to lead a team of engineers to success, has a vision for how tools should be built, and has an unwavering passion for developing content creation tools.
Previous experience working on first-person or other action-oriented games.
LEAD SOFTWARE ENGINEER, TOOLS
It's the same quote from above.
A third job has went up that asks to animate a wide variety of characters and creatures. A wide variety of "creatures" suggests it's probably not Overwatch, but the rest of the IPs seem plausible.