As for MOTU story content, they've been sneaking some Filmation-specific references in there (Zodac's bio is almost straight-up from Filmation, Buzz-Off and Mekaneck being pals, Saz being mentioned in Carnivus' ad, etc). Plus, there's inherently some crossover since Filmation (as well as MYP, etc) used Halperin's series bible as a basis and the bios are also using Halperin material. However, with Filmation rights, they could really expand the story by bringing in more specific information, artifacts, background material, and most importantly - all the cool original characters like Shadow Weaver, Icer, Stongarm, Scorpia, etc. I'd also dig Filmation-inspired designs of some MOTU characters they already have rights to, such as Clawful, Mer-Man and He-Man himself.
I doubt it'll happen anytime soon, but I'm hoping Mattel eventually comes to some agreement with ER or whomever owns the rights.