Harley will eventually betray Arthur.
Given the historically toxic relationship between Joker and Harley, it would not be unexpected for her to see that Arthur is out of control and might cross a line that even she thinks goes too far. The climax might see her shooting Arthur (she’s seen pointing a gun at him in the trailer) and escaping from him. Alternatively, Harley will be the more evil of the two.
Arthur may be the one who thinks Harley is too much for him, and he will either kill her off for good or abandon her and leave her in despair. Harley will kill Sophie.
She might mistakenly believe her to be Arthur’s ex-girlfriend after he runs into her again, leading to her flying into a fit of jealous rage and killing Sophie. An older Bruce Wayne will appear.
He has been following Joker and Harley’s criminal activity, as well as the rise of other criminals in Gotham. Bruce may not officially become Batman by the end of this film, but there might be a Sequel Hook for a third film in which the Dark Knight and Clown Prince of Crime square off. Another Batman villain may be alluded to.
They won’t physically appear but one could hear or see on the news something about “District Attorney Dent” or something of that nature.- Confirmed by behind the scenes photos in which a protester holds a sign up mentioning Harvey Dent. Whether or not he appears remains to be seen.
Mark Hamill will have a surprise cameo.
Bonus points if he plays Arthur's real father. The events of the movie will have all been in Joker's head.
The whole thing is how Joker views his sessions with Dr. Harleen Quinzel while stuck in Arkham Asylum. Arthur is actually the Supporting Protagonist/Decoy Protagonist of the film and the story is actually about Harley's Start of Darkness.
The film has specifically stated to have been given a Genre Shift to a musical with Harley played by famous singer Lady Gaga, there are a couple of scenes that seem to hint that Harley is a Loony Fan, and she's introduced in the first place as part of choir group with other inmates. Many of the scenes between Arthur and Harley are going to be imaginary in the same manner with Sucker Punch, while in reality, things happen completely different.
Based on the trailer, the scene of Arthur and Harley dancing in a ballroom or theater looks like it is a product of their imaginations. Harley is going to be revealed as Arthur's psychiatrist at the end of the movie, who only tried to cure him and allowed him outside like in Shutter Island trying to bring him back in touch with reality
Batman himself may appear...
...but since the film is told from Joker's perspective, we'll never get a good look at him or see who he is under the mask, allowing them to avert having to cast a big name actor to play him.