What's the real reason why the Joker is in this game?
I have one of two theories as to why Joker is once again the main villain:
1. WB Montreal wasn't confident enough to do a Batman game without him, because they think we buy BATMAN games for the JOKER.
2. It was planned that way all along. I remember reading details about the next Arkham game way back when. They said it was going to be a Silver Age prequel detailing Batman's first encounter with the Joker. But then they also said Superman, Wonder Woman and the Justice League would be in the game too, to promote that Justice League movie that will probably never get made.
It's one thing to include the Joker in this game, but it's another to build up other villains as the big bads and Joker as just a theatrical nuissance, and then pull a bait and switch! It was a very poor decision on SOMEONE'S part. Someone who, as I said above, thinks we buy BATMAN games for the JOKER. We buy BATMAN games for BATMAN!
This whole Joker craze has really outlived itself. I guess we could blame Heath Ledger for it, seeing as how he was SO good as The Joker in The Dark Knight, but that was 2008 and he's starred in THREE Batman Arkham games. Time to step aside.