Warner has outlined an Arkham Knight Season Pass that'll cost you £32.99 to gain access post-launch content over a six month window.
Details are few, but along with the expected new Challenge maps and character skins (for both cast and Batmobile) is confirmation of new story missions and additional super-villains "invading Gotham City" - note the two are listed separately, so it's speculation currently as to what form or gameplay the new (familiar?) faces will bring to the game.
You can buy the Season Pass separately, or together with a Premium Edition of the game, retailing at £87.97 for console, and £67.98 for PC.