I'm a bit of an oddball, so keep that in mind as you read my list.
1) The Unabridged Book of Furres: Rules for almost all the different varients of anthropomorphic and zoomorphic creatures, the relationships between the different species (as a guideline), different methods of handling mating (creatures can't mate, offspring are a mix of the two, offspring are one or the other, etc), some campaign ideas, and so forth.
2) The World of Gold Digger: For those of you familiar with Fred Perry's Gold Digger, this would be a rule book statting out the series, from the fantasy world of Jade to Gina's hurtbots. It would be a very rich setting to write into (and the art would, of course, be top notch - wouldn't want anyone other than the creator of the series to do it), and I think it would make a fantastic setting.
3) A sourcebook for the Rigante, another of Gemmel's worlds.
4) A setting based off of Sonic the Hedgehog, and my own variant, The Invasion. Though the series are admitidly childish, the core concepts in the comics and the series (excluding the Adventures of series) are solid and could create the base for a very dark setting. Assuming groups can get over the 'furrephobia' that some people seem to have...
5) A setting based on SpaceBalls. A parody, yes, but a very fun parody that would be interesting to play in every now and then.
6) A setting based on Isaac Asimov's works. Another rich world to play in that would definitely have many roleplaying opportunities.
7) A monster book dealing exclusively with myths and legends. There's more than one legend dealing with Vampires, so why not stat out more than one variant of Vampire?
Um... I think that's enough for now. I might add some more later, but maybe not.
Magius out.