In The Familiar, Realic made "Clip 4". In Real Life, Danielewski published the short Clip 4, writing as the fictional "Realic" from The Familiar, and in it, Navidson from House of Leaves is mentioned: "Very Navidson-y!" is an annotation on something that indeed calls back what happened in House of Leaves.
The House will be explicitly mentioned in The Familiar. Maybe even visited by characters. Cas seems the most likely candidate, as she's touring around the U.S. anyway and might as well end up in Virginia where the House is.
Or, one of Cas's Clips from the Orb is a scene from HoL, showing the Navidsons in the House.
Or, the FBI investigate the House, find something that leads them to L.A., then have to work with Oz.
Or, Realic had (prior to his involvement with VEM and his death) visited and investigated the House, to write an article about it (like he had investigated "Clip 4" and written an article about that) and that article by Realic about the House will be shown.
Clip 4 makes it clear that characters from The Familiar know about what happened to the Navidsons, so the House and everything that happened there exists in the reality/Universe of The Familiar. Since in HoL, at the end of the story the book itself is published In Universe, the House of Leaves book we know in Real Life, also exists In Universe in The Familiar.
Someone connected to the cat (Astair, Anwar, jingjing or even Xanther herself) reads House of Leaves, and realizes the spooky events in HoL are connected to the cat (see entry about the "Minotaur/cat connection"), or starts to wonder about what "VEM" is.
Johnny Truant will be a character in The Familiar. He lives in L.A., where six of the Familiar main characters live too.
He's most likely to meet Luther. Either Johnny is still a drug addict and is buying drugs from Luther (and/or arrested by Oz), or Johnny has gotten his life back together and now owns the tattoo shop Luther visits (after all, Johnny was originally an apprentice tattoo artist in HoL, and Luther loves to get tattoos).
Or Johnny needs therapy from Astair for his trauma from reading Zampanò's records and his strained relationship with his mother Pelafina.
Navidson also might show up. His family moved to L.A. and they need therapy for the traumatizing events of House of Leaves, and Astair becomes their therapist. Or he might be called in to do his job as a photographer, e.g. by Shnorkh's friend who's trying to document the Armenian genocide.
The evil creature of House of Leaves was called Redwood. In October 2017, Volume 5 of The Familiar will be published, its title is"Redwood", AND it's announced that in this Volume, the cat will be named. Putting two and two together: the cat will be named Redwood. And it is a manifestation, reincarnation or whatever you call it, of the Minotaur.
The Minotaur first was in the House of the Navidsons, then in Zampanò's house. But after Zampanò died, the Minotaur "bodysnatched" one of the many cats hanging around Zampanò's house (it was mentioned in HoL how many cats always were around Zampanò's house but they suddenly disappeared), and then wound up in Singapore at tian li's (the cat later somehow inexplicably insta-moves from Singapore back to L.A. where Xanther finds it, so apparently it has the ability to "teleport" itself; again, due to its magical nature of being the Minotaur/Redwood).
Would be even more brilliant if Xanther actually reads House of Leaves (the book as we know it in Real Life) In Universe and gets the idea to name the cat "Redwood" from that.
Volume 01 shows the tip of the tail, Volumes 02-05 the rest of the tail. As more books are published, they will show the entire cat if you put all books spine-to-spine.
Seeing how Shnorhk describes his interactions with his wife Patil, and his marriage in general, it comes across as if he sees her as a housemate and probably a friend, but he doesn't seem to feel any kind of romantic affection towards her. In Vol. 01 he states that they don't have sex (anymore?) which is fine by him. By Vol. 02, when we first see them interact, it becomes clear that he's actually annoyed by most of her behaviour. So... could it be that they just have a marriage of convenience? That might be either with or without her knowledge, actually. She does seem to be in love with him, judging from how caring she acts towards him in Vol. 02. Also, their back-story is briefly stated, which is that when they got together, she was coveted by a lot of other men but was head over heels in love with him; however, nothing is ever stated about his feelings towards her.
Even more WMG: Maybe Shnorhk is gay and for whatever reason, never came out and married a woman as a cover-up. Being a closeted gay would make him even more The Woobie than he already is—on top of the rest of the sadness in his life leading to his depression, he'd also be hiding his sexuality and foregoing his chance to experience true love. He could even be (secretly) in love with Mnatsangan. The way their interactions are described in Vol. 01, it certainly seems they share a deep affection, and a past which isn't fully revealed in the narrative... were they once lovers?
Each character is color-coded in the tips of their pages. Under that, they each have a stripe in different colors, some of which correspond to other characters' colors. It's complicated by the fact that some of the stripes don't really correspond with other characters', but a plausible theory is that the characters are / will be connected as follows:
(See [1]
for a more visual lay-out of this, and more explanation).
Anwar —> Cas —> jingjing —> Xanther —> Luther —> Isandorno —> Shnorkh <—> Ozgur
(with Astair pointing to Isandorno as well)
These connections seem to make the most sense, both color-wise (i.e. jingjing's stripe is pink, Xanther's color) and story-wise (i.e. jingjing has come to L.A. in book 4 to look for his lost cat, and Xanther now has it).
The Sorcerer recognizes Anwar and Xanther when he's looking into the orb with Cas at the end of V01. So he knows both Cas and Anwar. Both Mefisto and the Sorcerer are computer programmers. And them being the same person, would make it very convenient to connect Anwar and Cas' storylines (see the "characters crossing paths" WMG above).
- This also raises the question of how the prank-gone-wrong that Mefisto (Sorcerer) pulled on Anwar at the beginning of V01, is related to why Sorcerer (Mefisto) is on the run. Clearly something went very wrong with Sorcerer when Cas meets him; maybe the outing of Anwar's phone numbers was just a side effect of that.
- Confirmed as of Vol. 02's last chapter, where it's stated Mefisto and the Sorcerer are indeed one and the same person.
As of 2018 the series has been discontinued... or at least the form (printed books) it originally was released in has been. Fans have speculated that it could work very well to continue as a comic book series though (note that the books always relied much on visual representation, and that parts of the books, most notably the "Senex" sections, already were in comic book format). Or it even could be made into a TV series—especially given that the entire premise of the book series was/is for it to be "television in a literature format".
