Problem is that the term 'light' and 'CAS' generally don't mix very well.
If its light because they skimp on the armor, then it becomes significantly more vulnerable to ground fire, and hence loitering around to provide CAS isn't smart.
If its light because its a small airframe with limited weapons capacity..... well then I don't see how thats a benefit over upgrading the F-16.
If its light because it lacks a lot of the targeting equipment, all-weather capablity, and some crappy radar set, then you're really just shooting yourself in the foot by preventing it from being able to opperate autonomously.
I'd also like to note that really, nobody has a 'light-CAS' airframe, nobody really has a CAS fighter that can rival the A-10, and virtutally nobody has a dedicated purpose-built light attacker.
Now thats not to say I don't think the idea has merit, just that since nobody has one, the USA isn't at a disadvantage by not having one either. IMO, extend the F-16's wing span and load limit, add on more bomb racks, and call it good.
As for the F-22, I will say that it has a couple advantages over a cheaper figher like the F-16, or the Su-27. Its damn hard to spot on radar, which means it can attack from relative saftey. And second, it can act as an AWACS-lite. While the second doesn't nessicarily make for a good fighter, its probably going to have better survivablity than a regular AWACS due to the stealth.