Is not Alicia Silverstone, who else could've you seen play Cher?
I know that Reese Witherspoon auditioned and Kerry Washington was in the running to play Dionne. Reese would interestingly, go on to play a roughly similar character to Cher Horowitz in Legally Blonde.
https://ew.com/movies/reese-witherspoon-kerry-washington-clueless-audition/
Sarah Michelle Gellar was also supposedly considered:
https://www.capitalfm.com/features/stars-who-turned-down-movie-roles/sarah-michelle-gellar-clueless/
I wonder how Christine Taylor would've fared if you use her portrayal as Marcia Brady in The Brady Bunch Movie as sort of a template.
https://www.google.com/search?q=christine+taylor+marcia+brady&sxsrf=ALeKk014krzayi9wWts66D80b3AaSlRaog:1602491973613&source=lnms&tbm=isch&sa=X&ved=2ahUKEwjCrt7m067sAhUSA6wKHQvEAqUQ_AUoAXoECBcQAw&biw=1600&bih=789
Maybe if she were a few years younger, then I could've easily seen Kristy Swanson in that role.
https://www.google.com/search?q=kristy+swanson+buffy+the+vampire+slayer&sxsrf=ALeKk01MYWcBsBW6WJ458CGu-uYtpCC--w:1602492051554&source=lnms&tbm=isch&sa=X&ved=2ahUKEwj9vfOL1K7sAhVQiqwKHW0QAtEQ_AUoAnoECC8QBA&biw=1600&bih=789