So I actually had time to sit down and talk about the Pokémon Go meta and it's of a Pokémon not yet in that game yet. But you know what, let's speculate for the fun of it.
I would assume Greninja won't really be that great in Pokémon Go until at least they do a Pokémon Go community day. They've historically hold back on giving the starter's strongest moves (Hyrdo Cannon for the water starters) for the release of that event. I was going to say there's no way it could keep up with the raid meta, but then I remembered Greninja's base stats, and if I'm remembering where it stands compared to other water starters, Greninja has potential to be hype. Long story short, fast strong special attacker that will almost guaranteed to get access to the strongest Water move in the game will sounds like a winning formula to a fantastic water type to use in raids and parts of Trainer Battles where Combat Power(CP) isn't restricted. For more detail, keep reading.
Those who don't know, stats in Pokémon Go behave a little bit differently than the main series games. But instead of making up new stats they use the base stats of Pokémon from the main series games and essentially use a formula to translate them into the base stats Pokémon will have in Go. To dumb it down both of the attacks get combined into a single attack stat, both the defenses get combine into a single attack stat, HP/stamina is based of the original HP, and a part of the speed stat gets added to the attack stat. I bring all this up because this translation favors the spread of Greninja's stats tremendously.
For a comparison, Swampert is currently one of the best water starters in Pokémon Go. Only under Kygore & Gyarados for a water type attacker (not including shadow Pokémon because that's a whole other story) for the raid scene. Plus it has a fantastic nitch in all three Leagues of Trainer VS Trainer battles, even ranking within the top 10 in Great & Ultra. Comparing Swampert's stats to Greninja, the stats of the later favor the way stats work in Pokémon Go even better. It's frailty might hurt it's standings in PvP, but with how strong it's stats should translate in Pokémon Go I see potential for it to become one of the top water types as soon as it can learn Hydro Cannon. Probably being just under Kygore when at that point. And that's not even including the Ash-Greninja form which, I'm not even certain how they would implement yet.
As for Trainer VS Trainer/PvP battles. Way too much of that is dependent on what moves they allow Greninja to learn, so it's hard to accurately predict. If they translate even half of Greninja's diversity into Pokémon Go I certainly see potential in all leagues. Plus that water/dark typing might allow it to cave up nitches the same way Swampert did, if not flat out be a powerhouse. However, Pokémon with lower defenses tend to struggle more in PvP, especially in the lower leagues, so I could also see it being too frail to be useful there.
They're still rolling out gen 5 as slowly as possible, so it could be awhile but we'll see exactly where it stands once it comes out, more so once it gets access to Hyrdo Cannon. Doesn't help that Hydro Cannon is probably the most most of the three elemental moves, to the point where Ninatic nerf the power of it, specially for PvP, to help balance the game.
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