The next campaign will be the last in the series, so I suppose the devs already have something in mind to do after that, periodic events, new characters to unlock, new game modes; maybe a symmetrical PvP game mode or co-op competions... that'd be healthy for the game, since currently everyone complains about PvP being unfair. And survival challenges seems like an idea that suits the game very well.
In the short term they could do the events.