Bring back infection game mode. I enjoyed playing it back in halo reach. How about a slayer game mode with only the assault rifle and no shield. That might attract more of a audience who like games like cod...and maybe a ops mode like odst or even like the one in halo reach, i spent hours playing those would be nice to play with friends and have the option to revive your friend like in the campaign. Have a last boss like the boss in the campaign.
Btw in the ops mode have the option to use your cards for different guns etc..might have more of a reason to keep the cards to try to finish the ops mode.