Rendering power. Since the all 1P thing happens only in head to head setups, then it's using 2 arcade boards (the PS4 based System 478), one to render each scene. Presumably, everything off camera isn't rendered to keep things running at 60 fps the entire time.
These new features & characters are all good & well but....
Nina.....remember her? Blonde hair? Purple body suit? Had her own game? Likes to grind the pointy end of her shoe where it's not wanted (or is wanted), enjoys kicking men in the fruit bowl? Had a sister with a four letter name with two A's & two N's in it? Ring any chimes? ;D
Really looking forward to the new Iron Fist tournament but the lack of that four letter N word is really starting to make me just that little bit nervous. Other than that, game is looking superb & i can't wait to see what else that Harada fellow has planned. :D