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.
Talking Pokemon, the hacking scene's done more than you ask.
1. My hacks do everything but the Fairy type idea and Curse being Ghost for Emerald and FireRed. I don't really incorporate Fairy-Type into mine, because to get it to do much outside of natural type defenses I'd need to add some attack moves and probably add in damage split which would violate the hack's purpose. Pokemon Throwback also exists for FR/LG, takes a different approach to what I did with FireRed 251.
2. MrDollSteak's attack and decap rombase does all this mentioned plus a truckload of moves from the new games without replacing. Needs configuring by yourself though, one of these days when it's finished I may
make a version of FR251 incorporating this, but I'm not making any promises.
3. There's a method for just about every version out there for Fairy-Type without
replacing the ??? type (no need to be replaced really), if someone hasn't made a patch for it somewhere (I'm absolutely sure it exists where Pokemon hacks are common for everything but sapphire and LeafGreen because redundant) there's a tutorial for the DIY work.