TheStarTurtle I wouldn’t worry just yet. Katarina looks very much like she’s wearing a disguise in that scene to infiltrate a party she isn’t supposed to be at. As for LeBlanc, well, I can understand why they would want to eschew her “classic” outfit for something more appropriate to an immensely powerful clandestine sorceress than a magician’s assistant who strips on the side lol.
That said, LeBlanc has been confirmed as a master shapeshifter, so there’s no reason she can’t instantly sex herself up if she wants, and we know that Arcane isn’t afraid to sex things up when they want. They gave Vi a belly-baring alternate outfit in season 2. I doubt they’ll keep Kat’s midriff covered the whole run.