New Ultimate Skin Riddle Solved. "VI? stands for SIX!"
Just solved this and i must say that Riot was clever with this. Let me break it down. First we know that "6" was a major hint at the champ the skin would be for. Second and the giveaway was the Brand picture at the store currently having the 6 on it (with a circle and a highlight that STRONGLY resembles the gauge on Vi's back and gloves). Now Vi, if you follow her lore, knows almost nothing of her early years and no idea of her own true name (or she simply decided that she didn't want to give her name to anyone. The evidence is still inconclusive). The only clue that she had to identify herself with was the tattoo on her cheek (or BRAND if you will. See what Riot did there?), so the people who knew her simply called her Vi. But the mark is not Vi. Its "VI" which are the Roman Numerals for the number 6. Also, if you watch the Harrowing video there is another hint hidden in the video, that being the actual number 6 on the female puppets face much like Vi. Now i find it funny (and very clever) that Riot would make the big hint 6 as to make people say "what does the 6 stand for?" when Vi has multiple (3 i think) quotes that are simply "Vi? Stands for Violence!" and "Vi? Stands for VICE!" and "Vi? Stands for Vicious!". This again tells you that perhaps Vi herself has no idea what "VI" stands for. Another theory from the myths and theorys posts in the Story and Art board state that perhaps the gang she ran with identified its members with numbers as code names (this theory is much more logical and i encourage everyone to check out dialMARK4acti0n's post on family ties between Jinx and Vi. i will post a link to his post at the bottom of this one). If you were to truly identify her by the tattoo on her face then her name wouldn't be Vi, but VI (6). So the truth is folks that the new ulti will belong to Vi and now we know that "VI? stands for SIX!" and that 6 stands for Vi.
Quick Edit: Take a look at the Tattoo on Vi's left breast in her "Officer Vi" skin.
Edit: Another clue that has been given was that the champ would have an "A" as the fourth letter in one of their skills. Originally i figured this to be a way to throw people off as all champions have "Recall" but after a slight realization (and face palm) she fits this bill as well. Vi's Ultimate (level "6" skill), Assault and Battery. On top of this you can take it a step further and see that the "A" is the fourth letter consistently. ass"A"ult "A"nd b"A"ttery (yes that is the word ULT you see between those first two A's).
Edit: A new hint that was given a while ago was brought to my attention. "Assassin". Vi's Secondary class is Assassin.
Edit: A hint was given some time ago that was taken down soon after it was posted (tweeted?). The hint was "The Jokes on you." One of Vi's quotes goes hand and hand with this. "Here comes the punchline!" If you put the hint and Vi's quote together you get "The Jokes on you! Here comes the punchline!"
Edit: Link to dialMARK4acti0n's post on theories about Jinx and Vi. http://boards.na.leagueoflegends.com/en/c/story-art/lEN9q900-jinx-and-vis-family-ties-facts-myths-and-theories