Folks on NeoGaf have spotted the Mad Hatter in Arkham City, in a quick scene in the middle of an episode of Sony's Qore show. The Mad Hatter has evolved into a sinister mind control villain over the years. Hopefully he'll mess with Batman's mind in a similar to way to the Scarecrow's memorable hallucination sequences in Arkham Asylum. Poison Ivy is also spotted in the video, returning from Arkham Asylum.
Elsewhere, following a mention of Nightwing in the console achievements list yesterday, Comics Alliance have a screenshot that might well be him. Nightwing is a grown up version of the first Robin, Dick Grayson, who has had some truly stupid costumes over the years. The screenshot shows a man in a costume very similar to Nightwing's more recent appearance . See the images of him and Ivy below.
It looks as though Rocksteady are throwing the whole barrel of Batman characters into Arkham City. If they ever do a sequel, they'll struggle to fill it with well known new names. It will be obvious when they're run out, because that will be the day that they announce Batman's fearsome foe, Manbat as a playable character. That's unlikely to happen before Arkham City's released on October 18 in the US and October 21 in Europe.