These would be switches/toggles for custom games. Each of them could make the game more interesting, especially when combined. Usage of modifiers should disable the earning of achievements and significantly decrease XP earnings since these modifiers are for fun and some may be "unbalanced"
A few I thought of:
Moon Mode: The force of gravity is 1/2 what it normally is.
Bomb Mayhem: Bombs are the only items that spawn.
Tiny Titans: Players and weapons shrink to 1/2 their regular size, but the map remains the same.
Sudden Slumber: Every 45 seconds, players will fall asleep for 2 seconds.
Glass Cannons: Players deal & take double knockback.
Item Overload: Up to 10 items can spawn on a map.
Fog of Fury: Maps are covered in a fog that significantly reduces how far players can see.
Deadly Darkness: Maps are covered in darkness akin to the original Castleween map.
Gravity shift: Every 45 seconds, the direction of gravity changes
Blind Loyalty: Everyone receives the Face Off Pawer.