You're discussing impossible, because Petraeus, as CIA Director, was legally barred from getting involved in political campaigns, much more being a candidate himself.
Indeed, which would be even less plausible. Come on, a CIA Director abruptly resigning in order to join a ticket opposing the very President who appointed him and with whom he worked just fine? How would that play? Horribly, especially in times like this.
And yes. Petraeus doesn't seem to have ambitions in electoral politics.
