Obliviously, I mean aside from one that already have.
First and foremost:
Omega: a renegade time lord that the 3rd and 5th Doctor faced. Lost into a sort of alternate universe composed of anti-matter and had his bodycorroded until all that remained was his will (also always portrayed as a massive scenery-chewing ham, but that's not really a negative). A sort of tragic figure in that what he really wants is to just return to his universe of origin. Granted, it would be tricky to think of some new storyline for him that the old series didn't already cover.
The Ruton Host: the sworn enemy of the Sontarans. Only made one appearence in the original series. It would be cool to if one time the TARDIS accidentially plunked down right in the middle of a Sontarn/Ruton battlefield.
The Sea Devils: cause they're awesome that's why (though I'd be fine without the Silurians or the Myrka).