The most likely female choice is Bachmann but she's just too crazy. Choosing a minority accomplishes nothing, he's not going to get a significant amount of black/latino voters no matter what he does.
The advantage of choosing somebody like Giuliani is that he's a relatively moderate pick and the primary issue that he's concerned with is something that Romney hasn't had a chance to flip-flop on yet. If Romney wants to announce his VP right now, it's because he wants somebody who can distract from the Bain attacks but not somebody who will do a million Palinisms by election day.