3. Minimal Joker
At some point, we really need a full Arkham game without the Clown Prince, as there are only so many times you can repeat the same "They can't live without each other" stuff, before it gets annoying. Even Arkham Knight purported to be all about the titular Knight and Scarecrow, only for Joker to be in the game more than ever as a figment of Bruce's imagination.
In Insurgency, the cordoned off portion of Gotham the game takes place in is beset by an outbreak across Arkham and Blackgate, headed up by none-other than Joker himself. Apparently, Joker is embodying the Ledger-esque "I'm a dog chasing cars" approach to the situation, as it's revealed he helped orchestrate the breakout, but isn't answering to whoever's in charge of the grand plan.
From there, it's said Joker might even help out Batman and Robin if their motives align in the moment, but is there any character in entertainment history less trustworthy than the Clown Prince?