Are they really related to the cloudy-stormy-foggy-smokey teaser? I kinda doubt it since they are just so many and there is no way they can all be related. Also, what do you think is being teased and why those 7 champs? I personally hope it's Ao Shin, but I can't think of a way to connect him with the said champs...
Unidad2 votesHalloween skins.
Dreamspitter3 votesHalloween skins.
Yeah but
Xerath Cass Lissandra Zed
dont HAVE any Harrowing skins.
And 2 champs (Soraka/wukong) who ARE getting them dont have fog on their page at all
GenoXx1 votesThey are all portrayed as evil
The Mormonator2 votesI feel like it's the spooky ghosts 
GreenLore2 votesalmost all of these champs seem to have betrayed someone at some point. Xerath betrayed Azir Zed went against his order TF betrayed Graves Cassio betrayed Sivir Lissandra betrayed her sister(implied by gregor from the HA,who questions on which side she is) Leblanc is basically known for deceiving others And Hecarim is implied to be not that loyal to the king of the shadow isles.
Fusion3221 votesand tf is dark too, who already has a Halloween skin.
Garentheon1 votesHarrowing event
Cakelynn1 votesI've heard somewhere that its not Ao Shin, but another new champion. But that's just what other people have said. My first guess was the same as yours, or the Harrowing.
Avious1 votesI've heard somewhere that its not Ao Shin, but another new champion. But that's just what other people have said. My first guess was the same as yours, or the Harrowing.
I know about an avenger like female spear adc called Kalista from the shadow isles. Dunno why but im sure its not true.
Cakelynn1 votesYeah the same person who said it was Kalista also leaked gnar and aram so its probably right
Solideus1 votesSomeone linked an archived version of the TF-page. The altered splash has been there since july, at least. I guess the other's have been up for a while too. They might just be the wrong versions (Hecarim's just looks unfinished) or altered to fit the website's format (Like LeBlanc's?). I doubt the splash inconsistencies has any significance.
The fog is of course very new. The fogged champ's are all betrayers; it could be, among other things, the start of a champion teaser, or maybe new lore content. Who knows at this point.
Toa of Death1 voteswhen Ao Shin was teased part of his lore was that he would appear when Ionia was in the most danger. Maybe these 7 champs have something to do with a threat to Ionia. Or Harrowing
junglerboy161 voteshttp://boards.na.leagueoflegends.com/en/c/miscellaneous/Nov1wbBy-ao-shinshaco-rework-info-ive-found This guy put together a comprehensive list of the changes and some theories about what they may mean. It is definitely worth a read.