5: Hollywood
It is oblivious to no one that DC wants their own conjoined superhero universe to match or potentially succeed Marvels success. We also know that Hollywood loves money and that whether we like it or not, everything is done to make gangbusters at the box office. The Avengers currently holds the weekend box office record reeling in over 200 million dollars domestically. Do you really think Warner Brothers is shying away from a project like this? They most definitely want a slice of this pie while the market is abundantly available. Green Lantern was a critical and commercial bomb though denting future plans. Man of Steel may have split critics but it made money. It admittedly did not put up Batman numbers, but you bet your behind injecting him to Supermans universe would inflate that while effectively creating that much aspired to conjoined universe.