Mattel dropped a little hint on the Mattycollector Facebook page:
I know people can say that it could belong to Perfuma, but people on Facebook have begun speculating that the vine "friend" we have here is a small version of the Hydra that Evil Seed rode in the 200X Series.
Translation: The mystery Monthly Slot MAY BE Evil Seed's.
Why do I say Maybe and not IT IS! Because there have been rumors about a mini sub. As you may remember I already did speculations for those a while back. (200X, New Adventures, Mini Comics, He-Ro: Son of He-Man, Another Filmation, 200X Variants, Mini Comic Variants)
Damn! That's a lot of Speculation!
IF the rumors of a 200X Mini Sub are real, then He's a decent headliner for it... rumor mill has mentioned Veena and Sorceress as characters likely to be in it. Then again, they kinda make sense because:
Veena completes Preternia.
200X Sorceress can sit on the Grayskull throne better than Vintage Styled Sorceress.
So, in any case it looks like we're likely to get Evil Seed in MOTUC. (Just as long as they keep him 200X, of course, because I'm no fan of Filmation's Evil Seed)