I think a MvM mode with some extra features, like turrets or buffs could work. They've established a bunch of evil organizations such as Los Muertos, the gorillas from the moon, Talon or the Junkers, all of which would provide endless amounts of minions for some PvE maps.
As for singleplayer, they could make short campaigns for each hero. Simple enough to serve as a tutorial, challenging enough to have some achievements and rewards.