Blizzard is currently recruiting for a Senior Software Engineer, who will be responsible for creating vehicles "in a robust first-person engine for an unannounced game." If that last phrase sounds familiar, it's because Blizzard was recruiting for what's likely the same project in November last year.
The successful applicant will not only create the vehicles, but will also be responsible for their "handling features". Aside from that, there aren't really any other details which could help shed light on the unannounced project. The big take away is: Blizzard's next game will have vehicles.
This new listing follows one in October which advertised for a "generalist artist intern" to work on an unannounced project. The listing specified that a familiarity with Overwatch was preferred, which confirms my completely unsubstantiated suspicion that this unannounced project is some kind of Overwatch campaign off-shoot or, perhaps, a Destiny-esque PvE title.
Another listing in June also specified that its applicant needed to have "previous experience working on first-person or other action-oriented games." That's a solid year of development at least, but Blizzard is still showing no signs of making an announcement – perhaps it will at Blizzcon next year.