Since there's some talk about playlists, i'll give my two cents regarding them.
Core lists:
Ranked: Team Slayer, Team Objective, Squad Battle. Possibly Lone Wolves, depending on demand. Ranked games give extra experience in addition to the skill-ranking but don't allow guests.
Unranked: Social Slayer, Social Skirmish, Big Team Battle, Rumble Pit, Multi Team (4x2person teams).
Additional lists:
Permament additionals: Classic (objective and slayer), MLG (whatever the competitive community wants).
I'm not sure if SWAT and Sniper should be permament lists. Probably should, considering their popularity in Halo 3 and Reach (and 2?).
The rest would be rotated weekly, ie Griffball on week one, Zombies on week 2, etc. 4 of these would be a good idea probably, once per month each.
And then there'd be weekend lists, these would be experimental lists, versions of normal lists (BTB Heavies, Hardcore (limited lives)...), stuff like that, and Action Sack (though it could be a weekly list).
Max lists at time: 15, 14 without weekend list. That wouldn't be too bad.
DLC should be integrated on all lists, given slightly higher weighting and incentive to get them (double experience on them, extra Emblems for buying DLC or stuff like that).