Squats: I wouldn't want them to return in their original form, I think that should remain as a fond memory for old timers. But I'd be open to a complete reworking of the "space dwarves" concept
Harlequins: hard to see them working as a full army.
Cults: fluff-appropriate but lasgun-toting rabble doesn't sound a hugely inspiring concept for a totally new army?
Admech: maybe... if they can end up sufficiently differentiated from guard