Personally, I'd like to see stats for various real-world human analogies. Not even "Stats", per se, but just write-ups on Celts, Carthaginians, Romans, Greeks, Egyptians, etc. And I don't necessarily mean "mythology", which seems to be about as far as most books take things. I want names, culturally-specific rule tweaks, weird facts... the whole shebang.
If we're talking monsters, though... it's a tough question. There are so many good ones! I'll have to go with "Thri-Kreen", simply because I love Dark Sun and it's one of the harder races to convert on your own. "Half-Giant" would also work for the same reason.