There will be some sort of unlock-able items/abilities/secret things that will influence gameplay in Halo 4.
Without going into whether or not you agree or disagree with this change (such an argument would prove fruitless, it is already very much in the game), what do you think will be the kind of unlock-able things we will find in Halo 4? Keep in mind that these things cannot break balance, and cannot confer any net advantage.
My personal guess would be that many of them would be movement based, such as jumping a little higher or moving slightly faster. Someone who could jump slightly higher could play more “ninja”-like and possibly escape and ambush with this ability. Someone who could move faster may be more difficult to headshot and more difficult to kill at long ranges.