I think it would be Shadow Weaver.

Look at the line so far:

First Vintage character: He-Man (along with Beast Man)
First POP characters: Adora/She-Ra
First Horde Character: Hordak
First NA Character: Optikk (most popular NA character despite being a minor character in the original line)

They went with the heavy hitters each time. I wouldn't be shocked if they didn't do Shadow Weaver first, but since the demand is so high for her, I think they'd do it to have a good kickoff for the Filmation toys.