Something old, something new, something borrowed...nothing too blue.
I'd like the bases covered that the basic game didn't hit- maybe Oni of some kind, a playable fey gnome, and so forth- but some kind of system for generating balanced homebrew races and/or Arcana Evolved-style racial classes would be a godsend of a tool.
That would do for races what Green Ronin's Book of the Righteous (one of my favorite 3PP D&D products of all time) did for Holy Warriors.