What happened to Yachiru? According to the Wikia, Yachiru disappeared leaving her belongings after the fight with Gremmy. She is said to be "gone" by Kenpachi in the penultimate chapter of the original series.

I use "original" because there might be extra chapters announced as the other big announcement.

So, what happened to her? Was she part of his Zanpakutou or something?
The very last page of Bleach #668 gives a few clues.

Yachiru suddenly appears out of nowhere and says to Kenpachi:


   Silly goose, if you would just use me properly, there wouldn't be anyone you wouldn't be able to cut down.


She then touches him, which causes:


   him to get a power boost, which he wonders about.


She ends the exchange with explaining to him:


   That power is what everyone's been calling "Bankai".


All of these suggest the following:


   Yachiru is Kenpachi's Bankai. He is unable to call out his Bankai anymore (he was never able to, consciously - the original appearance may have happened subconsciously), therefore, Yachiru is gone.


Alternatively:


   Yachiru is his Zanpaktou spirit, again, one he has never been able to consciously communicate with. Though, I can't explain her disappearance if that were the case.