Here's a quick idea. It's based on two rules, so hopefully it's not too convoluted to be an OM.
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Dual-type inversion
-Monotype pokemon gain an additional type based on the first move in their moveset.
-Dual-type pokemon lose their secondary type and the first move in their moveset is changed to match their main type.
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The two rules are essentially inversions of each other, so hopefully their "symmetry" simplifies the concept.
Dual-type pokemon can lose their main type instead if the first move in their moveset matches their main type. The move will be retyped to their secondary type. For example, Stakataka (Rock/Steel) can turn itself along with any move in the first slot into a Rock type. However if the move in the first slot is a Rock-type move, Stakataka and the Rock-type move become Steel-type.
I think this would make for an interesting shakeup. It's clear that adding a new type is more likely to buff a pokemon than taking one away in exchange for a retyped move; however the majority of monotype pokemon are in lower tiers, and the majority of OU pokemon have great dual-types, hopefully resulting in a big tier shift and lots of interesting opportunities. Of course, the type removal option will still create some strong sets, but they may be more of a puzzle to find.
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Dual-type inversion
-Monotype pokemon gain an additional type based on the first move in their moveset.
-Dual-type pokemon lose their secondary type and the first move in their moveset is changed to match their main type.
---------------------------------------------
The two rules are essentially inversions of each other, so hopefully their "symmetry" simplifies the concept.
Dual-type pokemon can lose their main type instead if the first move in their moveset matches their main type. The move will be retyped to their secondary type. For example, Stakataka (Rock/Steel) can turn itself along with any move in the first slot into a Rock type. However if the move in the first slot is a Rock-type move, Stakataka and the Rock-type move become Steel-type.
Manaphy @ Waterium Z
Water ----> Water/Fairy
252 SpA / 252 Spe, Timid
-Dazzling Gleam ( adds Fairy type )
-Hydro Pump
-Energy Ball
-Tail Glow
Tornadus:
Flying ----> Flying/Fighting
252 Atk / 252 Spe, Jolly
-Superpower ( adds Fighting type )
-Acrobatics
-Knock Off / Taunt / U-turn
-Bulk Up
Noivern @ Choice Specs
Flying / Dragon ----> Flying
252 SpA / 252 Spe
-Boomburst ( becomes Flying-type. Luckily no -ate boost )
-Flamethrower
-Focus Blast
-U-turn
Stakataka @ Leftovers
Rock / Steel ----> Steel
252 HP / 252 Atk, Adamant
-Stealth Rock ( This matches Stakataka 's main type, Rock, so Stakataka will become Steel type and Stealth Rock a Steel-type move )
-Gyro Ball
-Earthquake
-Trick Room
Water ----> Water/Fairy
252 SpA / 252 Spe, Timid
-Dazzling Gleam ( adds Fairy type )
-Hydro Pump
-Energy Ball
-Tail Glow
Tornadus:
Flying ----> Flying/Fighting
252 Atk / 252 Spe, Jolly
-Superpower ( adds Fighting type )
-Acrobatics
-Knock Off / Taunt / U-turn
-Bulk Up
Noivern @ Choice Specs
Flying / Dragon ----> Flying
252 SpA / 252 Spe
-Boomburst ( becomes Flying-type. Luckily no -ate boost )
-Flamethrower
-Focus Blast
-U-turn
Stakataka @ Leftovers
Rock / Steel ----> Steel
252 HP / 252 Atk, Adamant
-Stealth Rock ( This matches Stakataka 's main type, Rock, so Stakataka will become Steel type and Stealth Rock a Steel-type move )
-Gyro Ball
-Earthquake
-Trick Room
I think this would make for an interesting shakeup. It's clear that adding a new type is more likely to buff a pokemon than taking one away in exchange for a retyped move; however the majority of monotype pokemon are in lower tiers, and the majority of OU pokemon have great dual-types, hopefully resulting in a big tier shift and lots of interesting opportunities. Of course, the type removal option will still create some strong sets, but they may be more of a puzzle to find.