Theres always DLC ;-) But yeah the scarecrow would be interesting to say the least. The concept of the character opens up many possibilities. Obviously, his gimmick would revolve around the fear toxin. I still can't make out what the jokers gimmick is in the fight I saw but he seems to rely on sheer insanity! LOL.
Although, I personally hope he is dead in the game, and that him being playable is only in flashbacks to the point in the story building up to his death.
I think the story would be more impacting that way, and in one of the endings it can result in the Joker being revived for the sequel or something.
i would love that ending. How bout this: a little ways through the credits it cuts to a scene of the league of shadows carrying a wrapped body and dumping it in the lazarus pit. You dont fully see the man that emerges, but Ras al guhl says something simple like "welcome back" and still without seeing the man you hear the jokers voice "glad to be back (insert maniacal laugh here)"