I've had a gantz oc only fanfic in mind for awihle now, but even though no characters plan on being mary or gary sue-ish I still don't know if a fic like that would be well received. Here's how it would start.
Synopsis: On Earth, only one Gantz team remains, and is under the sole control of Enkido, the last of an extraterrestrial race. The Gantzers become but mere pawns in a deadly battle against the being who designed it all. And with Earth as a bargaining chip, an alliance is formed between Enkido and Miles Lechaunce when the countdown to annihilation draws ever near.
And here's a more in depth look...
Story will kick off outside of Gantz with Miles Lechaunce and Takase Niito, each priveledged from multi-billion, dollar companies excelling in the mass production of vasts amounts of scientific advancements. Just before graduating highschool they both invent the Fountain of Youth Serum. However only one of them will be able to accept the accolades for this discovery and their friendship is tested.
Meanwhile, Niito's friend Elijah Kramarr and Lechaunce's sister, Dominique Lechaunce are secretly seeing each other behind Miles' back. They are then involved in a freak accident that places Dominique in a life threatening condition. Elijah begs Niito to use the Serum on her to save her life. Unknown to them, Niito's portion of the serum was compromised prior by none other than Miles Lechaunce. And even though Dominique's life is spared, the effects of the tampered concoction slowly turn her into something unhuman.
This leads to the fallout between Takase and Miles and inevitably results in their deaths, but unknown to them, fate would have it that in Gantz their bitter rivalry will be reborn.
Let me know what your take is, if made I plan on submitting it on fanfiction.net