I think that a properly new mode that is integrated into comp (so not like DM and CTF banished to Arcade) might have the capability of changing things up, along with ironing out some issues with ability bloat of newer heroes and the balance issues that comes with.
Is there a reason a CTF mode was banished to arcade?
Also a permanent game mode like Invasion from Halo Reach would be amazing. Only game that really did something like that decently, and they didn't expand on it much ever. Some of the most fun I've had with an FPS and something like it would suit Overwatch.
I don't think there's any sort of permanent Horde-style co-op mode either like Zombies in Call of Duty.
And somehow I feel like finally releasing a map editor would be Blizzard admitting defeat in some ways, which overall their hubris has prevented them from doing for a good number of years now. The company seems to think too highly of its development skills these days relative to the industry overall, which has gotten way better while Blizzard has stagnated or made little progress comparatively on the same timeframe.