With Overwatch becoming somewhat of a worldwide phenomenon since its launch, it's totally understandable that Blizzard would want another slice of that particular money-making pie. Judging by a job listing recently placed online, it looks like that next Overwatch project is currently in the planning and early production stages.
The position, a student-focused internship for summer 2018, will focus on filling the role of "Generalist Artist". While it's not officially confirmed to be an Overwatch project, the fact that the listing seeks candidates who have a "knowledge and understanding of the Overwatch universe" leaves little room for debate.
Blizzard often spends years and years developing projects, so there's no real clue when we will know what this project is. We also don't know the scope of the project, so it could just as easily be either a mobile game, or a full-on story mode. Personally, I'm hoping for something with some plot. I just love those magic science fighting kids so much.