Apparently everything has to have multiplayer, even Batman.
Kotaku has sources who say Batman: Arkham Origins will be the first in its series to add online multiplayer. Further details state that it'll be team-based with one group of players playing as members of a villain gang, and two other players taking command of Batman and Robin. Presumably, Batman and Robin will try to save civilians in danger while the villains must put an end to the hero's good deeds.
Although this is only rumored, it's in-line with the game's transition from Rocksteady Studios to Warner Bros. Games Montreal. The studio released Gotham City Imposters last year which was multiplayed-based with a similar focus on villains vs. heroes.
It's not a bad thing to try something new, especially adding play value, but a series as popular as Arkham must be treated with care. There's a whole lot that can go wrong, and only so much that can go right.