If it IS KOS-MOS
I'm going all in on a repeat of E3.
I'm not saying she's not hype worthy, but Sakurai's not an idiot. He knew that Hero would only really appeal to one half of the world while Banjo the other(generally speaking), so them being shown together was a win/win for all parties involved.
We know that production on FP2-1 started probably before the September announcement because, duh, and I highly, HIGHLY doubt they'd be only working on FP5 this whole time.
If it IS KOS-MOS, we're getting both her an an announcement of the 2nd Pass. It would make sense since Joker started the pass last time, in multiple meanings of the phrase, so this time KOS-MOS would 'end' it, while someone else would begin it anew.
That's my best guess.