Question mark is essential, because until we get an official word it could turn out to be baloney. However, the screenshots look fairly convincing, so I'll give them the benefit of the doubt. Anyway, someone managed to hack a Diancie, a Volcanion and a Hoopa on their team, don't ask me how but they did, and we got a preliminary look at their stats and signature moves. Remember, though, that we don't know the IVs and EVs on these Pokémon, just their natures.
First we have Diancie, who seems to have absolutely shitty HP and speed, passable attack stats and great defenses, with a seemingly solid signature move. Diamond Storm packs 100 power, 95 accuracy and an unknown chance to raise defense. Naturally, its worth largely hinges on the frequency of that random effect, since Diancie doesn't look like an offensive Pokémon all that much - even though it looks like both its attack stats could be base 100, since that 236 figure is exactly what a base 100 with perfect IVs and no EVs have. I wonder if I could try something like that on the other stats... more on this later.
Next there's Volcanion, which seems like a mixed bag in terms of stats, except for being quite slow as well. Its signature move is steam-based... what a surprise. Steam Eruption is a special Water move that boasts 110 power, 95 accuracy and a chance to burn. Once again, whether that chance is 10%, 50% or anything in between will greatly affect its usability, no doubt. One thing to note: I like how that "loop" of sorts on its back can separate into two steam shooters. Looks a lot cooler that way, that's for sure.
Finally there's Hoopa, who's clearly the most offense-based of the bunch, but yet again it's slow as ass - and look at that defense stat (with a boosted nature, no less). Ouch. Hyperspace Hole is another disappointment - it's a weaker Psychic that hits through Protect and Detect, and it looks as though it never misses, given that there's no number for accuracy. Now I'm sure it'll be useful in settings where Evasion Clause doesn't exist - no wait, that won't happen because event Pokémon are banned in these cases anyway. So Psychic and Psyshock could very well remain the more viable alternatives.
Anyway, can we pinpoint the stats on these guys with any kind of precision? Well... no. But here are rough estimates of what we could expect assuming those guys have perfect IVs and no EVs, as mentioned earlier, give or take a point or two:
Diancie: 40 HP, 100 Atk, 150 Def, 100 SA, 140 SD, 50 Spd
(HP is probably 50 and SD 150, read below)
Volcanion: 65 HP, 110 Atk, 120 Def, 120 SA, 90 SD, 75 Spd
Hoopa: 80 HP, 110 Atk, 50 Def, 140 SA, 130 SD, 70 Spd
These all add up to 580, whereas the standard for event Pokémon (Keldeo and Arceus aside) is 600, so it's unlikely that these are the real stats, especially since I did some rounding (not nearly enough to add up to a -20 point difference, though). Still, it's interesting to look at. For reference's sake, 580 is the BST for "trio" Pokémon (plus Keldeo), of which we've had none this generation. Maybe...?
Thanks to Looneyman for the tip.
Edit: It was theorized by many that Diancie would have a connection to Carbink somehow. Now look at the three non-signature moves this one has: Moonblast, Light Screen and Safeguard. These are the last three moves Carbink learns by level up. And since it's safe to assume I undershot Diancie's HP and SD by 10, this would mean it has the same stats as Carbink, aside from the higher attack and special attack. Were all these crazy theories about Carbink somehow evolving into Diancie... accurate? Or maybe we're getting a Phione/Manaphy relationship all over again?