This is only an assumption and should not be taken as fact, but I am certain that the Victorious skin will be teased this Friday, October 11th. The reasoning behind this assumption is because Victorious skins are always teased sometime within the first two weeks of October, but the reason why I specifically say Friday is because previous Victorious skin teasers have always been on a Friday. If for some reason it is not this Friday, then it is guaranteed by the next Friday, October 18th because it will be exactly 1 month before the season ends and Victorious skins are usually teased exactly 1 month prior to the end of the season.
As for who the Victorious skin will go to, it depends on whether there is a pattern or not. Within the past 5 seasons each Victorious skin has gone to a different role:
S4 - Support - Morgana
S5 - Bot - Sivir
S6 - Top - Maokai
S7 - Jungle - Graves
S8 - Mid - Orianna
If this is the trend, then this season will be a Support as they would be the next in line for a Victorious skin. However, the first 4 seasons did not follow this pattern as 2 Junglers and 2 Supports got Victorious skins 4 years in a row:
S1 - Jungle - Jarvan IV
S2 - Support - Janna
S3 - Jungle - Elise
S4 - Support - Morgana
If Riot wanted to even out the Victorious skins a bit more, then they would probably give the skin to either a Bot, Top, or Mid, as they are the only 3 roles that still only have a single Victorious skin. It is most definitely not Mid as they just got a skin last season, leaving just Bot and Top as the contenders. Between Bot and Top I am more inclined to say Bot as Victorious Sivir is older than Victorious Maokai and in recent times Riot usually likes to spread out Victorious skins between roles.
If the skin is a Support, then it would probably go to either Sona or Taric because of their domination in the early season which is around the time Riot looks for Victorious skin candidates. I'd also be inclined to say Rakan because he is an extremely consistent meta Support, but he recently got a skin as well as an eSports skin earlier this season and Riot usually does not give a Victorious skin to a champion that recently got a skin. If it is a Bot laner I'm not sure who it would go to because there are several meta ADCs that could be possible candidates like Xayah and Kai'Sa for example, but my personal biased pick would be Jhin because I am a Jhin main plus he is a very popular ADC among both players and Riot employees although not the most meta in pro play, but he is known for shaking up the meta a lot because of how certain items or Runes would just become broken on him due to his Passive (e.g. Guinsoo's Rageblade, Hail of Blades, Stormrazor, etc.) I also think Jhin would look really cool with a Victorious skin. His design in general just allows for some really cool skins.
These are basically all my thoughts on the upcoming Victorious skin. Let me know what you guys think and please feel free to leave your opinion as well.
TL;DR Victorious skin most likely being teased either this Friday or next Friday. Most likely will be a Support, if not a Bot or Top laner.