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...
almost 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.
I feel like it's the spooky ghosts 
Halloween skins.
I'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.
http://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.
Someone linked an archived version of the TF-page.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.
when 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
and tf is dark too, who already has a Halloween skin.
Harrowing event
