Rocksteady's Rumoured Justice League Game: 16 Features It MUST Include
7. Seasonal Content
The thing about games-as-a-service is that they're built as a base set of features that is supposed to be expanded upon as they go. While I've already called for entire cities and the heroes/villains who make their homes there to be released as additional content drops, these would likely be paid expansions, each with their own storyline. It's the free content that keeps players coming back to a game like this, remembering how much fun it is, and thinking about putting some money down for one of those expansions.
Seasonal content is a good way to do this. Have us take on a bizarre mission involving Mr Mxyzptlk for April Fool's Day. Give us Mister Freeze and Toy Man around Christmas. Perhaps Cupid is shooting the wrong sort of heart arrows around Valentine's. Maybe Calendar Man has something special for you on your own birthday. Taking a note from MMOs - these events should offer plenty of unique things to earn for players.
Not only should these sorts of events be released through the first year or two of the game's existence, but they should stay in the game's download file, automatically activating each year to lure players back under their cowls.