That would be a lot of work to implement though, wouldn't it?! You'd need reminders sending out that you have to challenge somebody (otherwise you may not BE challenged!!), I dont know, it just seems a lot!! Its basically a good idea, but you'd end up with the 5 best players continuously playing each other.
Which would solve the original problem of frauds at the top, but it might take the fun out of playing if you're playing the same people all the time.