The Secret of the Sword, but that was more of a spin-off than a crossover, though their Christmas special could technically count. So besides that, all we got was the Transformers episode Only Human where Cobra Commander makes an appearance as Old Snake.
Why were there no other cartoon crossovers? I understand that you had different toy companies and productions studios that kept toys like M.A.S.K., a Kenner product, from showing up in Thundercats, an LJN toyline. But there were several toylines produced by the same animators and toy manufacturers: GI Joe and Transformers were animated by Sunbow and owned by Hasbro. He-Man and She-ra were both produced by Mattel and Filmation, just like Bravestarr. You'd think characters from one showing up in another would equal more visibility, get kids interested in more and more toys. Instead, everything competed with everything else, no matter if they were all owned by the same people.
Revolution! While Transformers and GI Joe already had their own comics, the rest are joining them in a five-issue crossover event, which hopefully opens the doors for more crossing over between the titles. I mean look at that picture! How awesome is this going to be? Okay, so I don't know Micronauts or Rom, but I'm just stoked to see the culmination of what was once hinted at with Matt Trakker, the GI Joe figure.
Hasbro's plans don't stop with comics either, as they're creating a cinematic universe like Marvel's Avengers. Though it doesn't seem to include Transformers in it (which is a real shame), the other Hasbro brands will get movies that will conclude with a massive crossover, if all things go right. And since Hasbro usually has an animated tie-in to their on-screen properties, not including that abysmal Battleship movie, thankfully, there's a chance for new cartoons for most of these properties and a ton of potential for crossovers between them all.