According to the recent Nerdist newscast video embedded above, the Disney Channel's Dove Cameron (pictured above and at the top of article) is the odds on favorite to be cast in the role. Unfamiliar with her work, I can say that she looks like she would fit in nicely with the rest of the cast and would shake things up nicely.
And for those weirded out at the prospect of this superb night-time soap opera delving into the supernatural, this is nothing that the original comic book and the show's inspirational guide Twin Peaks have not done before. If anything, that is part of the charm the fictional town of Riverdale always had. Bring it!