Let's look back to why any given champion is chosen to get the victorious skin.
They have to be a Prominent champion for most of the SPRING split.
They have to be a champion that becomes prominent THIS season.
They can't earn the Victorious OR championship skins.
We see that that most likely canidates would then be Ashe, Varus, Karma, Jhin, Kha, Lee Sin, Graves. Ashe and Karma got the Championship and Conquerer/Challenger skins, and they've been popular power picks for a while, same with Varus. So next up would be Jhin and Kha. Jhin was just far too new, and had already gotten the SKT world championship skin from last year, so even though he was popular, he was a late Season 6 power pick, and thus disqualified. Kha and Lee Sin have been Meta Jungle picks for years, and so they're both disqualified. That leaves Graves.
Graves hadn't been a big meta pick in years, and when he got his rework with ADC's, his jungle power stayed constant for a long time and was one of the biggest jungle picks through the entire spring split. When I think Season 7, I think back to Xayah/Rakan (Who were both far too new to receive the championship skin) and Graves. I don't think about Jhin, or MF, or anything else, I think back to the insane power of the Healthy clear, potent tower taking, and raw damage of Graves Jungle.
Other possible Canidates from throughout the season are Cait, Galio, Taliyah, and Leblanc and Cass or Syndra. But as we can no other champion really fits.