I'm thinking the "Gender Key" is most obvious as one to come after the "Shadow Key", I have no idea how would you would make an "Animal Key", plastic or ivory, although that would be cool if done. As cheesy as it sounds, I would like to see a better "IDW Key" made(the first one being included with the IDW: 10 year Anniversary Set). I felt the "In The Can" story was vastly underrated for having Joe, Gabe and Jay work on together. I figure the key we won't ever be seeing is the "Race Key", because of it's complexity with other stuff on it. Pretty funny how it was originally supposed to be a cloak to achieve this too.
http://www.comicbookresources.com/?page=article&id=12517