12 Things You Didn't Know About Batman: Arkham Asylum
12. Mr. Freeze, Mad Hatter And Clayface Were Cut From The Story
When Rocksteady were crafting Arkham Asylum, they had no idea whether or not they'd ever get to make a sequel, and as a result they tried to cram in as many villains and characters they loved as possible. Of course, with the limited resources available, cuts needed to be made, and antagonists that couldn't be neatly weaved into the plot were saved for sequels.
As a result, Mr. Freeze, the Mad Hatter and Clayface were all either removed entirely, or relegated to cameo appearances in the final game. Clayface famously turned up towards the end of that original title, teasing his larger role in the sequel, but Mr. Freeze and Mad Hatter only received mentions in Easter eggs and Riddler trophies.
Although it would have been cool to have seen them, it's probably for the best that they were saved for Arkham City. Mr. Freeze especially benefitted from a larger role, and he could have very easily been overshadowed by that game's cast.