The canceled Blizzard MMO project that helped lead to Overwatch's creation was Titan.
Titan had been in development at Blizzard for roughly seven years before the company canceled it internally in 2013 and publicly confirmed the cancellation in 2014. The project was intended to be a large, class-based online game, but Blizzard concluded that it was not coming together successfully.
After Titan was canceled, a much smaller group of about 40 developers, led by Jeff Kaplan, was given a short period to develop a new concept. They built a prototype around the strengths of team-based shooters such as Team Fortress 2 and the cooperative structure of multiplayer online battle arenas. That experiment became Overwatch.
Overwatch was not simply Titan renamed. It became a new intellectual property with colorful heroes, an optimistic science-fiction setting, and objective-based shooter gameplay. Still, Titan influenced the project through personnel, early ideas, and reusable assets. This development history is why Titan is frequently described as the failed predecessor or creative starting point behind Overwatch.