in halo 3 you were rewarded for earning certain achievements why not bring this feature back along with some purchasable armors here are my ideas.
Security helmet- Get the audiophile achievement for halo 3 odst
Get the Marathon man achievement from Halo 3
Get the Dear Diary Achievement from halo cea
Mark V Helmet- Complete Halo Cea on legendary
Reach Milestone 40
Mark iv Helmet- complete Halo 3 on legendary
Reach Milestone 40
Eva Helmet- Reach Mile stone 50
Unlocks with the Avatar prop
Loyalaty / Green Flames- Complete Halo 3, Halo 3:ODST, Halo CEA, Halo Reach, AND Halo 4 In In At Least Heroic Difficulty.
Another idea is to change the theme of your main menu but to unlock it you must play the game required
Odst Black main menu- Play it Atleast once.
Silent cartographer Art Work main menu- play it atleast once.
And finally my last idea. Everyone knows that one of the reasons that make you continue to press on toward covenant hordes is the music, why not have music play when the game is almost over? that is all.