OK, so I remember last year Ishida Sui posted this on his twitter account on April Fool's day which everyone crossed out as a prank. The post was later deleted. Now I hear news about how a spinoff may actually be in the works! Does anyone have any news regarding this?
Can somebody PLEASE translate this page? Will be SO thankful!

Ok, this is pretty long and I didn't read the whole thing, but Shonen Jump + wanted Ishida to draw something for them. Then they somehow ended up talking about how Ishida is a fan of HxH especially Hisoka and what kinda story he'd create for him.
They got Togashis permission and the rest is a conversation between Ishida and Togashi about the name Ishida has drawn, which you can read if you click on the pic below.
So... nope, no official spinoff, seems more like a doujin. Togashi did say however that he'd like to incorporate some stuff when he draws Hisokas story someday.

Anyone with better knowledge of Japanese, feel free to correct me.

Here is what happened:
Tokyo Ghoul's Ishida Draws 69-Page Chapter for Hunter × Hunter Character Hisoka

Shueisha's Shonen Jump+ digital anthology and app released a 69-page storyboard chapter on Thursday drawn by Tokyo Ghoul original creator Sui Ishida that tells a past story of the character Hisoka from Yoshihiro Togashi's Hunter × Hunter manga. The Shonen Jump+ website also posted a dialogue talk between Ishida and Togashi that discusses why Ishida decided to draw a story about Hisoka, and Togashi's impressions on the chapter after he read it.
Ishida also posted a color image of Hisoka on Twitter on Thursday.
The chapter commemorates the both 33rd volume of Hunter × Hunter (which shipped on Friday) and the seventh volume of Tokyo Ghoul:re (which will ship on June 17).

https://www.animenewsnetwork.com/news/2016-06-03/tokyo-ghoul-ishida-draws-69-page-chapter-for-hunter-hunter-character-hisoka/.102821

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That clears up a lot. Hisoka having a past itself seemed so uncharacteristic of him, considering how Togashi goes out of his way to let us know how little it means to him. I had always assumed we'll never know. But it would be interesting to know Ishida's take on it. Thanks for the help guys!

Somebody's translated the interview and conversation between the two authors. It seems like Togashi considers Ishida's work to be a part of HxH universe and even talks about further expanding Ishida's story by writing about events right before where Ishida's start. [url=http://chiggix.tumblr.com/post/145336753292/冨樫義博-石田スイ-特別対談-少年ジャンプ]translation of the interview[/url]

Quote from alexonxanax

That clears up a lot. Hisoka having a past itself seemed so uncharacteristic of him, considering how Togashi goes out of his way to let us know how little it means to him. I had always assumed we'll never know. But it would be interesting to know Ishida's take on it. Thanks for the help guys!

Some ...

I second this and for me giving Hisoka a back story pretty much like open a Pandora box except at the bottom of the box is butt load of money made by Publisher.

I refuse to consider spin-off a canon because most of them suck and ruin the original series.