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I remember in elementry school, the other boys would let me play Harpies as such, since I didn't have Harpie Lady Sisters:

While you control 3 or more "Harpie Lady", when you declare an attack with a "Harpie Lady", you can increase it's ATK by an amount equal to the combined ATK of all other "Harpie Lady" monsters you control, and the effects of your opponent's Trap cards are negated until the End Phase. If you do this, you can't declare an attack with any other "Harpie Lady" this turn. When your opponent declares an attack while you control 3 or more "Harpie Lady" and the attack target is a "Harpie Lady", increase the ATK of that "Harpie Lady" by an amount equal to the combined ATK of all other "Harpie Lady" monsters you control until the end of the Damage Step.

Of course we didn't speak of it in official terms as above, but I've written it as such so it makes sense.

LOL, fun times. So, what kinda quirky house rules have you guys played with?

:v Fusions in the main deck? Huh...

Spontaneous idea for Fusions. Entirely new mechanic.

When you activate Polymerization,you can select a number of monsters on field/in hand to use a Fusion Materials. Stack those Materials together into one Monster Zone; they're treated as a single monster. This is treated as a Fusion Summon. That monster has all Attributes and Types of the Fusion Materials used. The Level of the monster is equal to the combined Levels of the monsters in the stack, and if it is higher then 12, it is equal to 12 instead. The monster has all effects of the materials in the stack. The ATK and DEF of the monster is equal to the combined respective ATK and DEF of the materials in the stack. If the monster would be flipped facedown, all materials in the stack are flipped facedown, and when it is flipped summoned, all monsters in the stack are flipped face up in the stack. When the monster is removed from the field, all materials in that stack are removed from the field.

Sorry, spasm. I've got game design on the brain. sweatdrop

EDIT: This actually seems a lot like Excess Summon. :v

Nothing crazy, but the decent local around here allows Japanese cards if they're in mass release English print.

Occasional proxy too.

Devastating Effect Sub-chains.

My Anubis-Reclamation combo can be quite the combo if pulled off. So I usually suggest this rule for similar combos that would have to be played with specific timing. Any attempts to counter the first effect/ensure it succeeds be placed in a sub chain.
Like for example. I play Judgment of Anubis in reaction to Heavy Storm. Should my opponent play Dark Bribe and I respond with Counter Counter, I couldn't play Trap Reclamation without making it the target of Counter Counter instead of my Judgment of Anubis.
With this rule the Heavy Storm/Judgment of Anubis chain would be set aside unresolved should my opponent have Dark Bribe or another countering card. A sub chain would then take place where me and my opponent activate traps to prevent/save Judgment of Anubis respectively. However the sub chain would only count if any attempts to thwart Judgment of Anubis were unsuccessful, if I was unable to play a card to stop Dark Bribe/Counter Counter then the chain would continue normally. If my opponent were unsuccessful, the sub chain would be resolved in my favor and we could return to main one where I activate Trap Reclamation. Any attempts to counter Trap Reclamation would be placed on the main chain. But if there was a third card in this combo, a second sub chain could be created as well.

It can seem like a complex ruling. But to my knowledge if my opponent were unsuccessful in countering my Judgment of Anubis, I could not chain Trap Reclamation to it successfully with other cards on the chain before Trap Reclamation and after Judgment of Anubis. If this is not true then my example was nearly useless. But it does explain the point of this rule that I have played by casually for years.