Week 5 – The majority of the city believes Joker to be dead. The City Council calls for an emergency election (Tetch’s work), and the ballot is set for a week after Joker’s ‘assassination’. And this, of course, is Joker’s endgame. Election turnout is the highest on record, bolstered by the fear and insanity of Mayor Joker’s reign. But Joker has been busy, and all the voting locations (public places, schools, all owned by the city) have been rigged with Joker Toxin canisters. As the crows hit their peak, Joker plans to trigger the devices, killing millions.
At the same time, Ra’s contacts Batman, and issues him an ultimatum. He has 48 hours to clean up Gotham, or Ra’s will clean it up his way. With Ninjas and swords and bodybags. Ra’s Deadline will expire on election night, in an attempt to put the maximum amount of pressure on the Dark Knight.
Even without Ra’s ultimatum, the Joker’s “assassination” and the subsequent chaos have left Batman and allies no choice; they have to act at once or Gotham will tear itself apart. They team up to take down the Black Mask’s gang, The Red Hood, the Darkwater officers and Lockdown, and all the rest of the Arkham inmates still at large.
Just as things look like they’re calming down, a message from Joker is delivered to the GCPD, now back under Gordon’s control, laying out his plan with the Joker toxin at the voting locations.
Batman will send the players off to disable the toxins and wrap up the rest of Joker's gang still at large while he goes to deal with Joker.
...and then the players end up fighting Joker anyway. Somehow. Haven't figured it out yet.
Like I said, rough draft. I'm going to change a lot as I go along, see what works, what doesn't, what plot threads the players show the most interest in. But it works well enough as a framework... hell, take this thread, get the DCA books, add water, and stir.
It'll keep you busy a few weeks, easy.