Anything and everything really, I'm a sucker for more monsters. Ideally though I'd like lesser known creatures/types to be brought forward, we have enough demons and undead to last a while, so things like fey and aberrations would be nice. Golems and constructs too, since they're really easy to include in any session where intelligent foes are present and they can be really creative in their appearance and tactics. Likewise it'd be nice to see things like Linnorms make a return, or other alternate dragon types that are different more on body-type than on coloration.
I've done my own conversions of 'em, but I love some weird, obscure monsters. Things like the penangallan, scalamagdrion, aurumvorax, amphisbaena, screaming devilkin, catastrophic dragons, strange oni variants, weird 4e style hybrid elementals, etc.