I would prefer the title 'Worlds Finest' just sounds classy.
As for plot: Darkside and the new gods invade Earth. That allows for enemies in both heroes weight classes. Lex Luthor is appointed as Secretary of Defence. He will be this movies Nick Fury and should be played by Clancy Brown.
He tracks down Superman and Batman easily because neither of them are good at hiding who they are. Batman should be played by Christian Bale as they havent established a new Batman franchise yet and a new guy is gonna confuse the hell out of casual fans.
Bruce redons the cowl after we see Robyn John Blake fighting a gang of looters. Blake hands back the cowl happily saying it was an honour to be Batman but just wasn't him. And also that it was a nightmare trying to learn advanced martial arts and how to use all the stuff in the Batcave. Because lets be honest the ending to Dark Knight Rises was kind of stupid.
He and and Superman can create the idea of Nightwing later.
Anyway at some point both heroes are captured by Dsaad and forced to experience the trajedies of their lives the consequnces their existance has brought to some and the reality that for everyone they save their are more they cannot. Than their emotions are played up and they fight each other. In some kind of Psychic world where niether has to depowered or impowered to match the other. It is a battle of wills. And they stalmate to show that they are both awesome and neithers fanboys have anything to bitch about. They agree to disagree on certain points and accept the realities of being a hero. And Bruce apoligises for abandoning his responsibilities at the end of his last movie which comes back at the end. Then they whoop Desadds ass.
Meanwhile the Joker having been broken out of prison in DKR is found by Darkside and given the power to corrupt anyone he touches making them as twisted as he is. But he is defeated by Blake the young idealist and then turns on the new gods because symbolism.
Superman, Batman and Luthor in a powersuit face Darkside. Batman sacrifices himself to save innocent bystanders (which helps teach Superman a lesson and set up for an HBO series of him being reborn through different eras of time.) and Luthor strikes the final blow on Darkside which leads to him becomeing president in the epilogue and announcing the formation of the Justice league.
Superman having learned about sacrifice and compromise for the greater good or whatever goes back to saving people while Blake becomes Nightwing to carry on Bruces Legacy while forging his own.