Like everyone so far, I'm generally in favour of a Reach-style unlock system. You play the game, you earn credits, you spend those credits on undersuits, helmets, attachments, skins, whatever.
However, I'd particularly like to see a much more modular style of customisation. For example, if you like the up-armoured versions of the Mark V and CQB helmets in Reach, you need to unlock the base version of both, and then the up-armoured versions of each. Under a modular unlock system, you'd spend credits to unlock the base versions of each, and then unlock increased armour as a single, separate thing, which could then be applied to any helmets you own in the same way as visor colours.
You could even have a tree of unlocks. A base version of a helmet might be cheap - twenty minutes of multiplayer, say. But once you own it, it unlocks another pair of helmets, and those two each unlock another pair of helmets. If you find one you like? You can spend credits to upgrade it, every upgrade allowing you to equip another attachment simultaneously. It's a good way to sink credits into things, and give players a real sense of progression as they unlock new items on the road to their favourite, or to make that favourite even more customisable.
There's so much you could do with this. Armour material options could let players choose between a glossy or matte finish; wear and tear could mean it looks fresh out of the factory or weathered and damaged; individual pieces might be cracked, or discoloured, sporting field repairs or emblems. You could have the shiny, chainmail-style undersuit from Halo 4, the softer, more fabric style from Halo 5, or even the more traditional, armoured ones from Reach. Attach an ammo pouch to your upper arm; a combat knife to your hip; maybe a tacpad on your wrist. Hell, attach a tacpad to every piece of armour! And why stop at just the armour itself? Maybe you want your energy shields to be red, or to make the classic noise while they're down and recharging, or your HUD to be purple? Perhaps you even want hard armour to appear around the edges of your screen, as it did throughout Halo 4's campaign?
And that's before we even begin to talk about monetisation. Let people just buy game credits with real money, or to immediately unlock a specific item at max rank, even if they haven't otherwise unlocked it.
Lastly - you think the current ocean of garbage in the REQ system is there because they honestly expect people to like it? Of course not! It's there to dilute the REQ pool, to decrease your chances of getting something actually worthwhile to frustrate you into spending more money on REQ packs. Under a system like this, there's no point adding crap since people just won't buy it and it doesn't hinder people from getting what they want, so we also get the nice side-effect of them having to produce high-quality items which people actually want to buy.