Once the Main story is complete, there is a Post Story, then after the Post story, more Difficult Dungeons Open up, Including Boss Fights with Almost every Legendary, and Once you beat a Legendary You can PLAY as them (Granted Because of how Super Mystery Works Each Pokemon that is not you or your partner has a Cooldown Timer after using them)

The Last Dungeon in the Entire Post-Game Requires you to have to have Every Pokemon Playable, Except for 1, You Also need to be at Max Rank too. The Final Dungeon is 99 Floors, and you are Forced to Change Teams every 5 Floors or so. And you cant Save Between Floors Unless you got the inventory Space for the Save Device Item.

Your reward for beating this Difficult 99 Floor Dungeon is you become Allies with Arceus, The God of the Pokemon World. And Arceus will also be Playable too.

There is a ton of Post-game Content in Super, so the Replay-ability is Pretty Decent, Which is something I appreciate.