Yeah, Halo Reach's ranking system could be better, it takes ages to level up and so many people just tried every trick and every glitch to get some onyx commendations..
But it's been confirmed that the Halo 3 system won't make a comeback, i'd say keep the Reach system, but make leveling up a bit faster and be more careful with the commendations.
Of course it'd be the easiest to just take out campaign commendations, since that was probably where the most boosting happened, but that wouldn't be fair at all towards Halo players without a XBL Gold subscription, they couldn't level up at all anymore. Well, really slowly at least