How short or long does the single player content seem to be? Are there any more unannounced fighters? And is there anything else hidden that we don't know about or is that it?
Well I couldn't play more than a hour by myself so far but the single-player content seems quite alright
You have the Ferrum League which is separated into the Red, Blue, Green and Chroma League which pretty much represent the difficulty it seems. You start at the easiest and each league is filled with pretty diverse NPCs, presumably their data is created by arcade players as each NPC has a distinctive edited design and a Win Lose statistic. You pretty much have to play tournament qualifiers first, then the actual league's tournament and then you can do a promotion match to advance into the next league.
Then you have quite of things to unlock to customize your player avatar and you can also unlock new outfits for Nia (the advisor woman). There are also 13? I think pages of titles you can unlock for your online profile, some of which require you to raise a Pokémon to S Level 100 so I can imagine that will take a bit of time along with the regular arena and support-set unlocks.
I didn't spot any roster additions but the menu-design definitely allows for more Pokémon than we have seen. Shadow Mewtwo can be unlocked with the NFC Amiibo Card but he only stays unlocked till you restart the game, you have to use the card again upon next boot, I presume till you unlock him normally via gameplay.
The tutorial seems to cover the basics very well and after I completed the initial three tutorial courses I unlocked two additional courses called the "Pokkén Special Training Course" and the "Pokkén Maniac Course", according to the game for "High-Level Players" and "Pro Details" respectively but I didn't try these yet. Other than that there also seems to be a Move Training course and a Combo Challenge for each Pokémon.
Other than that, as said the options feature a English/Japanese voice toggle and I already tried to do a online match and it seemed super smooth, no lag whatsoever was noticeable, this was against another German player. If they game doesn't find a opponent in 10 seconds after starting a search you'll be automatically put in a "VS CPU" battle but I'm unsure if they game actually continues to search in the background while you fight the CPU.
Local 2P battles also run very well despite the 30fps downgrade, we had tons of fun yesterday, also because each Pokémon's movelist is quite varied and you have tons of options. Graphically the game is nice with varied arena's and lots of details going on in the background though I noticed a strange blur filter layered above the Pokémon once I was home. This might have been a weird quirk of my monitor though as I didn't notice anything of that sort while we still were playing on my friends TV.
If there's anything more I should answer just go ahead and ask
Hopefully your friends won't chicken out like mine, we played Pokkén for two hours till we went back to Mario Kart Wii