Blizzard is currently working on an a first-person engine for an unannounced project, according to multiple job listings on the company’s website.
A listing for a Lead Software Engineer states Blizzard is seeking someone to work on "a robust first-person engine for an unannounced project."
Though vague, the job description provides a few other hints, suggesting the new engine could appear on "multiple platforms" and seeking a candidate with experience working on games that are "action-oriented." A separate Lead Software Engineer listing that appears on Blizzard’s site also mentions work on this new engine.
Source: IGN PC Articles