A GREAT PARTY SYSTEM!! I.E. 20 player parties

Everything you said basically. I'll add a couple.

- Really in-depth character customization

- Tons of weapon attachments, combinations, and upgrades. Things like scopes, sights, grips, etc.

- Many, MANY different game modes. The usual Team Deathmatch and Team Survival (no respawn mode) won't cut it. There needs to be original ideas.

- Splitscreen like you said would be AMAZING! Thats one of the best things about Halo 3 and Gears 2 online.

- Like you said, recording video/pictures would be amazing.

It needs to live up to it's name. "Massive" Action Game

Progressive persistant battles. By using extremely long maps the objectives would change depending on how well a team is doing and how close they are to the enemies final base. This would allow for extremely long battles that you would be able to quit and rejoin a couple of hours later if the fight is still going on. Matches would go on indefinatly until one team manages to destroy the enemy HQ. (Kind of like how Warhammer online's world pvp is setup)

;)