I'm not sure that I "need" a lot of extracurricular figures guest-starring in Masters Classics, but a few notables might not be too horrible. [^)
Blackstar is a good choice for inclusion in MoTUC, owing to his sneak cameo in a FILMation He-Man episode!
Marshall BraveStarr would not be out of place in a line that also includes Rio Blast. And I can just envision a team-up between Skeletor (or Hordak?) with that psycho Tex Hex!!
I'd personally be SUPER-psyched for Adam Power and Arkus (and maybe Shaya?) from Revell's "Power Lords" to be released in scale to MoTUC. My sister's Power Lords figures were ALWAYS crossing over into He-Man's adventures at my house back in the day. [^D
Galoob's "Golden Girl" is so much like She-Ra, it's uncanny! G.G.'s band of "Guardians of The Gemstones" are fierce and beautiful female warriors, and her conglomeration of adversaries (Dragon Queen! Wild One! Vultura! Ogra! Moth Lady?) are every bit as formidable.
While much (all?) of the supporting cast of Olmec's Sun-Man line are a bit far-fetched (even for He-Man?!!), Sun-Man himself is a great-looking, pretty iconic hero. Instead of Pig-Head (???), a guest-star cameo from Butterfly Woman (Sun-Man's She-Ra) would be very cool.
...It's easy to get carried away when deciding to shoe-horn entire other Toy Lines into the Toy Line You love. :^)
Mattel has its very own "Universe" of pre-existing properties that would be ripe for "Come out of retirement, You're getting a He-Man figure!" treatment. Characters from Major Matt Mason, Big Jim/P.A.C.K., Pulsar/Hypnos, Wheeled Warriors, Mad Scientist, Captain Power, Computer Warriors (?), Max Steel, even The Guardian Goddesses would all be mind-blowing inclusions. The Trick is to NOT get carried away, as I so obviously have. [^D
Maybe just a "Mini-Sub" of 6 or so high-profile choices..?