Various hints to the hero were present in the first cinematic released. The gauntlet of Doomfist is encased in glass, and on the wall are three variations of the hero: the Savior, Scourge and the Successor. The latter, shrouded in mystery, is sporting a different version of the gauntlet, as it seems to progress with each generation of Doomfist.
The gauntlet was placed at the Numbani museum as part of the Unity Day celebrations commemorating the peace between humans and omnics. At one point, Widowmaker and Reaper tried to steal the gauntlet until Winston and Tracer popped in to stop them, with the help of two kids as well.
There also seems a new Doomfist or possibly the one Winston beat – and took the gauntlet of – running about in the world, as there isn’t peace between humans and omnics everywhere. This is assumed due to a ComicCon panel held by Blizzard.
Plus, Blizzard previously said that should Doomfist become a playable hero in the game, he would be well aware of those that came before him and take up the mantle – for good or ill remains to be seen.
It’s fun to speculate about it at any rate. Especially since Blizzard is one of the biggest teases on the planet when it comes to announcements.