Subraces I like to see are the Genasi and the Ghostwise Halflings. Additional races I would like to see playable are Kobolds and Spellscales, as they are described in Races of the Dragon, and Warforged and Changlings from Eberron.
Some classes and races from the Magic of Incarnum and Tome of Magic might be interesting to play but do to their wildly different magic system I think they would be out of the scope of an expansion.