Respectfully, I have to disagree. Spinning Quagmire off onto his own series would be a terrible idea. Quagmire is a supporting player who's best taken in small doses. As a horny sex hound, Quagmire is neither compelling nor likable enough to carry an entire series. If Quagmire were thrust into the center, his character would have to be radically revised, resulting in a completely different Quagmire than the one that fans love, making the move counter-productive.
...And that's precisely why there would be no point in doing that whatsoever. Stewie and Brian don't need their own show because they already have one. Family Guy can survive without Cleveland, but not without Stewie and Brian.
Also, DarthGonzo is correct. Seth MacFarlane didn't choose Cleveland for the spinoff at all. Mike Henry (the voice of Cleveland) did.