Just one question: What kind of change are we talking about? Transform? New ability? Because there really is no way to "change" based on the opponent outside of those.
Well, A new attack or ability, or transform, are pretty much the only things that could do this yes. But that's still not very defining. And it's not as though I'm saying "The ability/attack has to do specifically -this-", I've left it rather open-ended.
OK, I'm revising OU DITTO, Should I deletethe old OU ditto to post this, or is leaving it there fine?
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Name: Shapeshifter
Description: A pokemon that drastically changes it's appearance/attacks/type/stats/SOMETHING (evolution line???) based on the opponent's active pokemon.
Now, I think there are a lot of ways this could work. First of all, this transformation could be an attack or an ability. Second, Almost anything could be changed. A few examples of shapeshifter pokemon are these:
Ditto/mew, transforms appearance,attacks, and stats, based on the opponent with an attack.
Kecleon, Changes type when attacked using an ability.
Porygon/2/Z, using download, changes into a special attack/attack sweeper depending on the opponent's weakness using an ability
Luxray/Nidomen, With the ability rivalry changes it's stats based on the opponent's gender
Gardevoir/porygon/2, gain a new ability using trace to copy the opponent's
I think there is severe wiggle room in designing this idea.
Just one example of something that we could make using my idea:
(this is just an example.) A ghost type shapeshifter pokemon with a new attack called "Fear". It works like transform, but instead of transforming into the opponent, it transforms into a pokemon with the best stat spread for combatting the opponent (150 Atk if the opponent has a lower defense, 150 SP atk if the opponent has a lower special defense, etc.), changes it's type to the type that is super effective against the opponent, and gains 3 extra moves of that type.
Now, I know that the above is extremely inplausible, (Not to mention a pain in the ass to code I presume.) But it just shows what kind of variance is availible with my concept.
How would this effect the metagame? Here's how.
Shapeshifter requires a counter that would not work in other situations. If every other team carries an otherwise-useless Shapeshifter counter, it will create more team variety. Now, carrying a complete counter, a pokemon that is absolute shit otherwise (Male combee, etc.), is a little bit extreme. However, carrying a pokemon that can deal at least a little damage, may be a safe bet. so if every other team carries a sorta counter to CAP6, that means one out of twelve pokemon will be a counter, such as a BL/UU pokemon. This would add a great amount of variety to the metagame.