What items are missing besides gems and Soul Dew (which it seems possible is in the game somewhere via a Lati@s-triggered event)? Gems don't have a huge effect in the meta, so I agree that if it's just that, not worth making a separate tier.
Event Berries (Custap, Rowap, Enigma, Jaboca, whatever else), Soul Dew, Latiosite, Latiasite,
Light Clay (it's a Golett/Golurk held item), Fairy Gem, all the other Gems besides Normal, some Pokéblock-only berries and... I guess that's it.
One further thing is unreleased Pokemon / megas, assuming someone with the ability to hack ever releases the stats.
MegaLatios, MegaLatias, Diancie, Hoopa, Volcanion. Not much.
Btw, how do we know the Hidden Abilities for Pokemon whose HAs aren't yet available (RBG mentioned Adaptability Adaptability Dragalge)?
Battle Maison and iirc Battle Institute. And even then, we don't know all of them (still waiting for Snow Warning Articuno and Drizzle Suicune...).
About PokéBank... I'm not a tiering leader but, personally, I'd rather we just disallowed items unavailable in XY and called it a day. That way, the transition will be easier once the app's finally released. I say "transition" because I also believe XY OU should be the official metagame, only for it to receive all of the Bank's additions when the tier comes, and the Bank metagame be renamed to "Unreleased" if there's enough demand. Actually, we shouldn't even have named them "XY" and "PokéBank", to begin with, but the deed's done already.
Problem is, no one cares lol. Everyone can see from Antar's stats that PokéBank metagames are much more popular than XY ones. Everybody knows the latter won't last long and many favorites (mostly legendaries) aren't available there so why even bother. This is why, practically speaking, we should just let the Council and C&C work with the assumption PokéBank more or less resemble the new, true XY metagame we'll have after Christmas.
Whatever stat change we have will certainly be a mere +10 increase somewhere, whatever new move will have as much a impact as any other special event from older gens, whatever typing retcon we get--oh wait no old-gen legendary is Fairy-type because Fairy sucks. While I don't believe absolutely nothing will change, I'm pretty sure there will not be anything groundbreaking.
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I don't really know what should be done with the names now, but more importantly, if C&C leaders and contributors say they would appreciate this six-week headstart, then by all means they should write their analyses with PokéBank OU in mind. I think they really would be the most affected by whatever changes will come in December, should we keep XY OU as the official tier until then.