Yeah, I think Filoni was initially reluctant to bring Maul back, but it came from George.

I also am not sure if George wanted the Jedi Killer to copy his look from Vader, but it does seem like the Jedi Killer was meant to be the son of Han and Leia (which was revealed to the Visualists in Summer of 2013). It is possible though that was a Michael Ardnt idea. But the Jedi Killer being the grandson of Darth Vader at least rationalized the character’s desire to emulate Vader, even in appearance.