How about a hack/patch of Pokemon Ruby, Sapphire, Emerald, FireRed, and LeafGreen with the following Gen V and Gen VI changes:
The addition of the Fairy-type (replaces the ???-type; is unaffected by Dragon-type; deals 2x damage to Fighting-types, Dragon-types, and Dark-types, despite none of the changed moves dealing damage; deals 0.5x damage to Fire-types, Poison-types, and Steel-types, despite none of the changed moves dealing damage; takes 0.5x damage from Fighting-types, Bug-types, and Dark-types; takes 2x damage from Poison-types and Steel-types).
Clefairy and Clefable are Fairy-types.
Jigglypuff and Wigglytuff are Normal/Fairy-types.
Mr. Mime is a Psychic/Fairy-type.
Cleffa is a Fairy-type.
Igglybuff is a Normal/Fairy-type.
Togepi is a Fairy-type.
Togetic is a Fairy/Flying-type.
Marill and Azumarill are Water/Fairy-types.
Snubbull and Granbull are Fairy-types.
Ralts, Kirlia, and Gardevoir are Psychic/Fairy-types.
Azurill is a Normal/Fairy-type.
Mawile is a Steel/Fairy-type.
Charm, Sweet Kiss, and Moonlight are Fairy-type moves.
Curse is a Ghost-type move.
All Pokemon names, option names, and item names have mostly lower-case letters except for the first letter in each Pokemon's name (I.E.: Mr. Mime instead of MR. MIME, DeepSeaTooth instead of DEEPSEATOOTH, ViceGrip instead of VICEGRIP, Save instead of SAVE, etc.).
Mawile's misspelling in its Pokemon Ruby entry (as well as its incorrect footprint) is corrected.