
first off, partial genderswap: Ryou and Malik (sir-not-appearing-in-this-particular-au) are female -> Ryoko and Malikah
- The invention and development of Duel Monsters occurs earlier than in canon, so that essentially the cast of DM is in the setting of GX (Duel Academy et all)
- The main cast of DM are all students in DA. Ryoko's father gives her the Millennium Ring as a going away gift when she heads out for the first year of school. It causes (unbeknownst to Ryoko) quite a few strange disappearances...
- After the Spirit of the Millennium Ring challenges Yuugi (and fails), Ryoko becomes aware of the Ring's actions. Concerned, she tries to throw it in the ocean to get rid of it.
- The Ring promptly reappears.
- Ryoko runs away from school because she thinks it is the only way to protect people from the Ring. While in Domino, she accidentally finds her way into the seedier part of town and is tricked into joining Underground Dueling, which is a circuit of dueling that involves putting the duelists in a large cage and putting shock collars on them that go off as Life Points drop (see GX episode 65 for more info).
- Ryoko stays both because it keeps the Ring occupied (who loves it) and because those involved rarely let duelists leave.
- As one can imagine, the Underground (and other closely associated illicit activities that bleed into it) are not very good physically or emotionally for someone. Essentially, it breaks Ryoko.
- After a few years of this lifestyle, Ryoko has a few too many intense duels in a row and the last one is too much for her, so the loss of the duel also kills her.