Pokemon Concept:
Mechanically, what would you think of a Dark-type Pokemon that functions like an “anti-Zoroark?” What I mean is: imagine a Pokemon that has high stats on average, but no/few moves it can learn or be taught/bred on its own. Instead, it has a unique ability that works like the move version of Zoroark’s Illusion, where it fills in any empty move slots it has with the moves from those slots on the last Pokemon in your party? This is why I’m calling it an “anti-Zoroark”—while with Zoroark it always has Zoroark moves but hides under the illusion of another Pokemon, with this “anti-Zoroark,” you always know the type of Pokemon it is, but it could come at you with almost any move, based on what’s in the party with it, and generally has the stats to make them work? Weirdly enough, it’s a Pokemon where forgetting moves is actually *encouraged* (more empty move slots to fill with its ability). I get that this is a super gimmicky idea to base a Pokemon around, but is it an *interesting* gimmick, or just dumb?
hmmmmmmmmmm
Without more details, my first instinct is that this is too strong – like, if it has good stats and a bad movepool, but its bad movepool doesn’t matter because it can have practically any moveset you want, then that seems extremely good to a degree that is probably dangerous. The fact that its type coverage is going to be redundant with something else on your team is a limiting factor, but it could also have other tricky bull$#!t that just doesn’t rely on fighting toe-to-toe; maybe it has Spore or something, which on a fast Pokémon has a lot of potential for abuse. Hell, maybe you just put Smeargle in your last slot; then “anti-Zoroark” can have literally anything you want, but on a body with actually good stats. That’s terrifying. Of course, you can’t ever make a Pokémon forget its last move, so this thing is always going to be stuck with one move of its own. If its natural movepool is really $#!t – like if there is nothing good in there whatsoever, not even a good Dark-type STAB move – you could design a Pokémon that effectively has only three moveslots, which might be interesting. Without actually diving into all the zillions of possible combinations of moves out there, or doing the calculus of whether this is good enough to be worth having a Smeargle on your team with a moveset designed for a high-statted Dark-type, I really don’t know where this falls from a balance standpoint. And, of course, in the post-Sword and Shield era, maybe you can just never allow this thing to exist in the same game as Smeargle; that’s probably safest. Still, my instinct is very much to assume this thing will be super overpowered unless someone can really convincingly prove otherwise.
Pokémaniacal 
I could see an Unown Evolution/Form working with this ability. The only thing I’d add is that it could only copy moves that matched it’s Hidden Power type. So a Psychic one might have: HP: Psychic, Psyshock, Miracle Eye, Hypnosis/Calm Mind. A Fire one could have: HP: Fire, Sunny Day, Flamethrower, Fire Blast. They would need boosted stats of course. With the type limitation you could even get away with taking moves from multiple Pokémon. You’d need a complicated hierarchy, though, or grab moves randomly, like Assist.
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I forgot to add that it wouldn’t have access to any normal moves like: Protect, Swords Dance, Shell Smash, Belly Drum, Boomburst, Double Team, Minimize, Explosion, Self-Destruct, Guillotine, Horn Drill, Lock-On, Extreme Speed, Odor Sleuth, Foresight, Heal Bell, Soft-Boiled, Morning Sun, Nature Power, Pain Split. Just some examples.
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Really like that idea!
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Possibly broken, but an interesting flavor of broken (compared to say, Dracovish, Gorilla Tactics, Zacian).
Instinctively I prefer the idea of a Pokémon that’s merely decent stat-wise, but incredibly versatile – but my vague impression of say, Smogon tiers and so forth is that versatility in itself isn’t that great. (Versatility is a huge advantage for a Pokémon that already has at least one thing it’s really good at, but jack-of-all-trade-style Pokémon seem to sink through tiers, despite their versatility, until they land at a tier where they’re the best at… well usually it ends up being U-Turn and Stealth Rock/Defog?)
So I think, assuming that “high stats on average” means something along the lines of 500-550 BST (not 600+), this thing won’t be a nightmare to deal with. The fact that its versatility comes at the cost of team redundancy might even make it kinda underwhelming (Mew has a great movepool and base 100 across the board, and IIRC it hasn’t been a consistent Uber, I think it’s in UU every now and then?).
I agree with pokemaniacchris that Smeargle is a problem. I think the official tournament format is Doubles where you bring a team of six and then use four of them? Less (but not zero) opportunity cost to using Smeargle as the moveset medium for anti-Zoroark there, compared to 6v6.
(WeedleTwineedle would have a field day with this Pokémon.)
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This could be a Bug type parasite pokemon, somehow needing to attach to the last pokemon on the party to “survive” without harming it, but using 3 of the moves. I would try to make it frail like a Shedinja or Pheromosa, extremely frail pokemon but could make problems for the opponent, with diverse levels of success. Or maybe it could be the other way around, like a Shuckle. Maybe even the stats of the “host” could define a form, a variant from attack or defense style of parasyte. A serpentine tapeworm pokemon, feeding on attacks, could make this.
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