Since this is the terrible idea thread, I'm pro-chaos and pro-spectacle. I'm not all that interested in fairness or the sanctity of the game.
1. Have each team send out both offense and defense.
2. Play starts simultaneously on both 30s.
3. No field goals.
4. Whoever scores first wins.
5. If you turn it over (on downs or otherwise), the opposing team gets the ball at the same 30 going the same direction. This means that one team could have both possessions at the same time.
6. If a fumble recovery or interception return crosses the 50, all 44 on-field players are eligible to participate (if they aren't otherwise occupied with their own play).
6a. If a play crosses the 50 into an active play, both plays will continue. If both plays score, whichever play scores first wins.
6b. If a play crosses the 50 into a dead ball, all on-field players can participate. The offense is free to snap the ball to start a play as well.
7. Players can switch ends of the field as they please between plays, but not during plays (except for cases where the ball crosses the 50). One side can't start with 10 players and sneak an unknown player from the other side. If you have both possessions, you can either use a backup QB or one QB on both ends as long as you don't get any delay of game penalties.
8. 50 yard line exception: if the offense has possession while the ball crosses the 50 (say the QB is scrambling), players must maintain the integrity of their side unless a turnover occurs. This means that teams with both possessions couldn't intentionally take a big loss to run extra offensive players against an undermanned defense.