I was running around the other day, and I had memories of a game I used to play that built on "rules". These rules varied from good things (mostly) to some bad things which affected players, and sometimes NPCs. They had an event which would randomly select a few, or immortals of certain ranks could activate them. After the initial phase they were put in, they added a timer so the immortal could set the rules with random, and it would create a random timer. Of course, the bad rules generally had a much shorter timer.
But ill give some examples of what they had, and you will get the idea
Golden Goose: Double gold from all NPCs
Deaths Door: No loss from hitting a DT
Double XP: No need to explain
Quad Damage: NPCs and PCs both did 4 times the damage.
Adrenaline Rush: All things that can regen do it x4 faster
Blood Feud: Members of clans at war will do double damage
Bloodlust : Auto Wimpy Disabled
Bonded Artifacts: You dont drop a corpse when you die
Bullseye: Ranged and throwing weapons do X4 damage.
Cloudwalk: All Players and Mobiles can fly
Divine Blessing: All Hps, Manas, Moves, Vitality are x4
Divine Sanctuary: All Damage is halved
Mayhem:All players become PKable (no corpse looting) longest living player recieves bonus to regen and damage
Enchanters Delight: X8 mana regen
Eternal March: Teleports, summons, gates etc do not function
Fog of war: All exits are obscured and scrying is rendered useless
Gravity well:Movement costs are x4
Hellfire: When in direct sun you regen -800%, -400 in partial
Illiteracy:Scribe and recite functions fail to work
King of the hill: Like mayhem, but only valid for PK players, corpse drops like normal
Lock: Prevents rules from being changed except by a high lvl immortal
Never say Day: If you fall below 25% health, you will do quad damage
Night of the living dead: All npcs are considered undead
Nightmare: Damage to mobiles are halved.
No hesitation:This switches the mud to real-time combat instead of turn-based combat.
Nowhere to hide:When Nowhere to Hide is active all peaceful rooms are no longer peaceful.
Outrage:All mobiles are meta-aggressive.
Over Population:Zones respawn twice as fast
Povery:Mobiles give half gold
Prodigy:50% xp bonus
To the death:prevents fleeing
Underpopulation:Zones repop -50% fast
War of Attrition: PVP damage reduced by 50%
They all may not fit, but these are what they had. Some ruled, some sucked, but when a good ruleset went up people took advantage.