Originally Posted by
Simca
The timeline they gave in the Overwatch: Origins panel does suggest that this came out of the original Titan project. Additionally, when they interviewed one of the devs, the caption said "Lead Game Designer, Team 4". Team 4 was the Titan project.
I think the timeline is like
~2007: Team 4 formed. Project Titan begins.
~Summer 2013: Project Titan overhauled and taken back to the drawing board. Officially though, the team splits into two groups - one to investigate the possibility of refocusing the Titan project's core and taking its main idea forwards. The other group is tasked with creating another game inside the same universe that Titan was going to be set in. Each group has a new codename (we know that Titan wasn't called Titan for at least the last year, based on dev statements).
~Summer 2014: The group tasked to investigate taking the core of Titan forwards is disbanded, and the employees are redistributed from Team 4, mainly back to World of Warcraft (Team 2).
~Fall 2014: The group tasked with creating another game inside Titan's universe announces Overwatch, a team-based first person shooter.
So I think the universe that Overwatch is set in probably came out of Titan, and probably the engine, but the core of what Titan was is not related.