Overwatch players keep finding Doomfist in all kinds of places - just about everywhere but actually as a playable character. User crash reports from the Public Test Realm (where Blizzard lets players try out upcoming patches for balance and stability) were marked "18.104.22.168 Doomfist / Summer Games" until a recent update cleared out the title, leaving only the version number, as reported on Reddit.
It could be that Blizzard just snuck that reference in somewhere hard to find as a way to troll datamining players. But even the studio's jokes usually have a kernel of truth in there somewhere. And quickly going back to patch out the title adds some more weight to the idea that it really was an internal moniker that was never meant to be seen by the public.
If you're not familiar with the Legend of Doomfist, here's the brief version: the hero's name has shown up in a couple of places in-game and in surrounding materials, framing him as a powerful hero or villain (depending on who you ask). Also, Terry Crews really wants to do the voice acting for him. The last new Overwatch hero was Orisa, who joined the game back in March, then Sombra in November 2017 and Ana in July 2017.
If Blizzard maintains a roughly three-to-four month gap between new heroes, that would mean Doomfist is due sometime in June or July. Granted, there's no guarantee that Blizzard actually will stick to that schedule - it's never made any promises to that effect and Blizzard is the original "when it's done" studio.
Also, hey! Another year of Summer Games sounds cool. It would be be the first ever second annual event for Overwatch so I'd be interested to see how much would change and how much would stay the same. I'm up for a few more rounds of Lucioball, at the very least.