Just as a general nitpick: maybe some weapons and armor in a light "bley" scheme? The dark grey currently matches the old colors well, but not so much for the post-grey-swap elements (although, granted, the "metallic" colors work just as well, I admit). On top of that, some part of me is just a little nostalgic for the old light grey armors and weapons from the original Castle releases . . .
As for other colors: dark blue, red, and green might be handy for pauldrons and some armor, if only for that "painted armor / tournament" look, while GitD might work well for "cursed" weapons. Trans-colors in general might also be nice for wizard staffs (magic-imbued items?) and the Powered Assault Commando visors (in case anyone wants to integrate that design outside of Halo-themed creations), while trans-neon-orange for weapons would give them that nice, "fresh-outta-the-forge" look.
Beyond that, the only other thing I can think of is maybe try making a "rust" color for some items; some things might just be relics or artifacts, after all.
I like the rust idea it would look great on some mocs. Always in addation if you do decide to use bley you should keep the old drk grey around for some time, as a am half and half on the idea, i like bley on figures but hate the idea on regular lego bricks.