Ubisoft have teased Operation Grim Sky, which will be the first bit of new content in Rainbow Six Siege's new season, featuring two new operators as well as Hereford Base map rework. Oddly enough, the content wasn't leaked previously.
Operation Grim Sky's operators will not be from a single counter terrorism unit, but according to lore, they will be joining the same unit within Rainbow - Urban Tactical Response Team, codenamed Grim Sky (GSUTR).
One of them will be a crowd control specialist, since the official post describes her as a British "tough police officer who's an expert on mob behaviour and snatch-squad tactics". Her mind is set on being on the frontline apparently, which makes her sound a lot like a British version of Caveira. Maybe she'll beat the attackers with a baton until they talk, instead of just threatening them with a knife.
The other operator is an attacker, from the US of A. He seems to be an army veteran or a former CIA spook as the site states "he has seen the worst in Kabul" but "fell in love with the city" regardless. There isn't much info on this attacker, other than he is "legendary for his surgical precision" and that "he remains an enigma within Rainbow Six". Joining Rainbow suddenly feels fun, they apparently don't poke into people's privacy when recruiting them.
Either way, those two will not be the only additions for the new season's start. Hereford Base has been scheduled for a rework for a while now and Ubisoft stated that even though it's an "iconic training facility", they will look to rework it in order to "raise the bar for competitive play".
Perhaps Ubisoft kept a tight lid on this operation because it will be one of their big reveals during Six Major in Paris from 17 to 19 August 2018. This is good news since Gamescom will start a week later and Rainbow Six has already been confirmed on Ubisoft's roster so they will likely show off more gameplay and offer additional insights into Operation Grim Sky during the event.