I'm pretty sure I've read a story earlier (on Escapist too, most likely) on how Overwatch was actually put together from the pieces left over after scrapping Titan, so Titan itself isn't a complete enigma. It would've been set in the Overwatch universe, or something close to it, my personal bet being the Omnic Crisis period, seeing how it has an extensive group of "heroes" involved, with numerous faceless mooks to farm (it's mentioned in Bastion's backstory it was a very common mass-produced combat model during the crisis, one of the mainstays in the omnic army) and large scale boss battles (d-va's backstory mentions huge squid-like omnic creature attacking Korea iirc).
Edit: Here I even ferreted out the relevant [url: www.escapistmagazine.com/forums/read/7.947344-How-Overwatch-Was-Born-From-The-Ashes-of-Titan]link[/url].
Edit 2: the url bbcode seems borked, here's a raw version of the link: www.escapistmagazine.com/forums/read/7.947344-How-Overwatch-Was-Born-From-The-Ashes-of-Titan