And how does 343i incorporate a similar investment system without completely changing what Halo is.
The whole investment system in CoD is based around unlocking stuff that you can use in game to [potentially] be more effective.
Based on the hatred of AAs by many in this community, you really think the notion of hidden, statistical perks would go over well?
Or spawning with sniper rifles or rockets in custom classes, instead of fighting for power weapons on the map?
Well Halo is probably not going to have you unlock weapons and/or perks as you rank up, but that's not where I think the focus needs to be. The investment system in Reach is oddly backwards, prioritizing time you play the game instead of rewarding players for their performance. One of the things that COD and Battlefield do so well in-game is constantly show players front and center what they are doing and giving them points/xp/CR.
If you use Reach as a base, greatly expand commendations, and stop hiding points you get in the corner of the screen without telling you what you are earning. Get points in the center of the screen for kills, headshots, assists (variable assist points based on damage), assassinations, you team holding/capturing objectives, sprees, multikills, include large amounts of points for your team winning the game.
Tie all of those to commendations or something similar so that you hit milestones as well. Also simply the point totals you are granting players. For example, make a kill 100 points/cr/xp. A competitive multiplayer kill in Reach without any medals is worth 4 cr, which doesn't strike me as particularly enticing to the player.
As you won't unlock the ability to use or spawn with better weapons similar COD, you should still be able to unlock cosmetic things as you progress. Armor, weapon skins, logos for your service record, new HUDs, etc.
For me, a good player investment system is not about unlocking things, but making the player feel rewarded for their actions in-game. I always find it very satisfying to see the points rack up for a huge variety of actions on the screen as I play.