Update 1: David Anders return has been confirmed.
TVLine has since confirmed that Anders will in fact reprise his role as Dr. Whale (aka Victor Frankenstein) in an upcoming episode of Season 5
Source:
David Anders teased this during a Twitter Q&A.
Thanks to Tyler for the heads up.
The
Doctor
Will
Be
In
#OUAT https://t.co/tgPRhmfQ9H
— David Anders (@QuestionAnders) September 29, 2015