.hack//Quantum is a three-episode OVA by Kinema Citrus and Bandai, created as part of the .hack franchise.It's 2022, and The World is still as popular as ever. Three friends — Aida Asumi, Ikuta Iori, and Etou Eri - are, naturally, avid players of it. During a regular dungeon crawl, they accidentally ruin a guild's attempt to kill an infamous monster, The One Sin, and promptly get a bounty placed on their heads. During the ensuing chase, Aida happens upon a strange cat PC by the username of Hermit, who's looking for something...It comes to a head when the chase lands them somewhere strange - an anomalous area called the Digital Prison, where Eri and a few others are PK'd and sent into a coma. Asumi and Iori are left to puzzle together why CC Corp is transferring comatose patients to one hospital, what a list containing the personal ID of the afflicted means, and who's behind the attacks...
.hack//Quantum provides examples of the following tropes:
Call Back: Reiko Saeki known as the player character Pi appears in a significant role.
Chekhov's Gun: In the background, one of Asumi's school lectures mentions that the real world has progressed technologically to the point where microchips are implanted into people's bodies to store personal data. Besides reminding the audience of human RFID chip implantation, this becomes a plot point when Hermit gathers players' personal real life data through these chips.
Cut Lex Luthor a Check: The technology to trap minds in virtual reality has finally been put to practical applications, being combined with cryonic suspension for storing criminals with life sentences and people with incurable diseases. The Digital Prison still has a few bugs that need to be worked out, though.
Human Resources: According to Geek, the caged humans in the giant tree are being used as components of a giant quantum computer, and each one contains 14 billion entangled qubits, which is about the number of neurons in a human cerebellum.
Tomato Surprise: Hermit flips out when he overhears out his real body been dead since 2015.
Subverted: And then it turns out he's not dead after all, and that his real body was only put into cryonic storage.
Double Subverted: According to the novelization, no one has ever been revived from cryonic storage, and the technology to do so will not exist in the foreseeable future.
Triple Subverted: But cloning from a preserved body is not technically impossible, and if the process that traps players in the game were reversed to put a digitally preserved spirit into a healthy young body...
Virtual Ghost: The new bout of coma victims, along with the old Dot Hackers' data.
World Tree: A corrupted version, the Digital Prison.
You ALL Look Familiar: Sakuya, Mary, and Tobias use (modified) avatars of Kite, BlackRose, and Balmung, respectively.
Hermit's avatar is a chibi version of Mia/Macha.
Shamrock's avatar is a variation of Aina's.
There is even a Player that looks like Orca in Episode 3
If you watch carefully, you'll see expies of Shino/Atoli, A-20/Natsume, Silabus, Kuhn, Grein, and even a female version of Zelkova. And sometimes even multiple expies of the same character in the same screen, but then again, it is an MMORPG...