It seems like a very George idea. I’m still skeptical (at best) of it, but thinking on it there’s probably a way to do it that’d be really cool and actually benefit the thematic through line of the trilogy and saga as a whole. If he’s just the big bad reborn again, that’ll be silly. But if he’s more of a specter, if it’s more about the long shadow that he cast and maybe his spirit is playing some role to carry out his legacy, it’d make sense if that’s one of the final sort of challenges that these characters have to overcome.
Nothing in Lucas Star Wars Saga supports he would have resurrected a character that way. Maul wasn’t his idea. It’s a very JJ idea on the contrary (i.e. trying to look cool while making silly movies such as TFA…).
It was however an idea during TESB post-production to make the Emperor the “last level” bad guy in Episode 9. By killing the character in ROTJ he changed his mind. JJ knows of it, maybe Kasdan reminded him of this abandoned idea. Since Kasdan is no longer the great screenwriter he used to be (see the terrible scripts he wrote recently: TFA, Solo), could be a possibility.