The ranking system(s) should give us a few options. For instance, Truskill would affect our rank in “Ranked” playlists and gametypes, while wins would affect ranks in “Social” playlists and gametypes, and Credits (Cr) earned with offline games, campaign, custom games, firefight, and Forge would raise a “Casual” rank (couldn’t think of a better name, lol). This would make everyone happy no matter how they prefer to play. The rank-based achievements would be unlockable by any of the 3 ways. You would unlock the same armor (if this is how you unlock it) by any way of ranking up, but the highest rank of each category would unlock unique pieces of armor.
Also, there should be “Unique” pieces of armor you unlock for each Halo game you have all of the achievements for (CE:A, 3, Wars, ODST, Reach, and 4). Unlocking all would create an “epic” set that all the pieces would match up and all that stuff.