Back in the 1990s, players of Starcraft, Warcraft and Diablo would all get dropped into a common meeting area in the battle.net lobby. This was genius design, as it created cross-pollination between the games. Starcraft players whom otherwise might not give Warcraft a second look would buy the game and give it a chance if only to play with friends who did. When Blizz segregated the games into their own ecosystems, the populations of all the titles were damaged.
Blizz needs to return to this successful model. Create a game hub for all Blizzard titles, this time a real city where players can walk about in the avatars of various games. They can show off their stuff to others who don't even play those games, which will lift all boats and generate new interest across the board. Sort of a walled garden idea. An Overwatch player can wear their epic skin standing next to a night elf priest in their top end tier set. They can chat and jump into games seamlessly.