I think once you hit the cred cap all xp earned from challenges and comendations should still be given to you. I say this because they are one time things and it takes a while to earn some of them, then once you finnaly earn it you get nothing.
if youre implying they are adding ranks/specializations anytime soon, they arent
this game will have a netlife of 1 year and when halo 5 comes out splat noise