It looks like the CW is already moving forward with the spinoff series of Riverdale, The Chilling Adventures of Sabrina.
Following news that it's currently in production on the network, Production Weekly is now reporting that a pilot is set to film in Spring 2018 in Vancouver. It's unclear whether the episode will serve as a standalone pilot or a backdoor pilot which will be launched on the original series.
Also, there's this tweet from Riverdale boss teasing a script page of the spinoff:
Stay tuned for more updates. In the meantime, you may check out the audition footages for the role of Sabrina below: