In order to design new content for a game, you must know what its design pillars are (we won't talk about art pillars, as they are not relevant to this topic). We can't know for sure what they are for Overwatch, but we can assume that at the core of the game experience there are things like: teamwork, fluid tridimensional movement, character diversity and uniqueness, and fun to watch.
Everything you design must relate to the pillars. In our case, this means that, obviously, the new character should be unique and diverse, but in a way that makes her (or him) interesting to play in regards to her:
- Movement style
- Specific role and function in the team
- Plus an interesting background that fits with the Overwatch ...