Archie's costume is the same one he wears in the Archie Comics.
Toni and Cheryl's Halloween costumes are Harley Quinn and Poison Ivy, two villainesses/antiheroines from DC Comics' Batman stories and frequent partners (in crime and otherwise).
Veronica mentions her friend Katy Keene when talking about getting costumes. This is in reference to Katy Keene (2020), the new CW show premiering in 2020, set in the Riverdale world.
Betty is dressed as Laurie Strode from the 1978 movie Halloween; and the episode is named after this iconic movie.
First mention of the character Katy Keene. Which will be a new spin off series airing in 2020 starring Lucy Hale And Ashleigh Murray (Josie McCoy)