They’d be shooting themselves in the foot with an OW hero that ISN’T Reinhardt or Winston. Reinhardrt who’s pretty much the only OW hero that fans REALLY want and Winston, who, if Tracer is considered the face of the franchise, then Winston would be both the brain and the voice.
Otherwise, Imperious would be solid though idk if he’d bring players back but it would get current players happy. Personally, i like Arcturus, but once again i don’t think he’d get people back so much as make people still here happy. Deathwing would definitely be a release that could get people back, but idk if people feel like waiting through another WC character and I’d like to Deathwing would be a release they’d save for a big announcement like Gamescon or Blizzcon.
Personally, and as much as i hate to say it, i feel that only a WC character really has the power and hype behind them to be a reason to come back to the game since it just has the biggest Blizzard characters. Deathwing, Grom, Mannoroth, Vol’jin, Cenarius, Vashj, WC just has all the big names. But if we’re thinking alternatively for big characters in the other franchises i can think of Imperious, Baal, and Zoltun Kulle from Diablo, and Arcturus, Swann, and maybe Selendis from SC, but like i said there isn’t a whole lot of groundbreaking heroes they can release from the other franchises like there is for WC.