In a post on the Battle.net forum Game Director Jeff Kaplan has dropped the bomb shell that they have Public Development Tools on the Roadmap, meaning that one day the public would be able to create their own custom content and maps for game.
Yes. We would love to make more of our tools available to the community. Our main issue has been that we made a brand new engine for OW. The tools and tech are not in a state where it is easy to make them public-facing. We keep talking about ways to get a roadmap to get us there someday. Both our design and tools team are very invested in trying to make this happen someday. But this concept would not happen anytime soon.
Though this may be a time off, this gives budding developers time to plan out and mock out levels in Model Editing Packages such as the Hand Painted Overwatch Inspired Japanese House made in Unreal Engine 4 (UE4), created by Callum Roberts.
This leaves us wondering if the new content will be added in the same way that new content is added to Team Fortress 2, with Staff Picks being added to the Rotation. It will be very interesting how it will work, as Overwatch has a much more embedded competitive system.