Thanks to Green Lantern, we know the planet Melmac, in the Adromeda galaxy, exists in the DC Universe - but it turns out it also exists in the Marvel Universe!
In the '80s, Marvel published an ALF comic that managed to run for 53 issues. 
In the '80s, when companywide crossovers were just gaining popularity, Marvel tended to restrict theirs to the books' Annuals, rather than interrupting the main story; the 1988 crossover was "The Evolutionary War," in which the High Evolutionary undertakes several simultaneous efforts to accelerate human evolution, meaning lots of different heroes could fight him in lots of different ways without having to, say, gather in one room and talk about it first; it's hard to conceive of a threat that both the Silver Surfer and the Punisher can deal with, y'know? Anyway, ALF Annual #1 got the "Evolutionary War" trade dress and the High Evolutionary showed up in two of the contained stories, warning ALF not to interfere with his plans for humanity. And also theorizing that Phoenix was the one who destroyed Melmac (though ALF said he had never heard of her).
Although it's framed like it could be a dream sequence, the final issue of the comic had ALF storming into Marvel's offices to demand it be declared real, leading to him getting his own Official Handbook of the Marvel Universe entry:
This is probably not the place to bring this up (especially since I haven't watched Guardians of the Galaxy 3 myself), but I do wish this was how High Evolutionary was introduced to the MCU. Just showing up and chatting with other characters for a bit, partially to show he's an intellectual and partially to flesh him out, y'know?
I like how he remembers giving a commencement address for the Melmac Orbit Guard when ALF mentions it, and actually is curious enough to ask about how Melmac is doing. It kind of reminds me of his encounter with Apocalypse, and how the two of them discussed their whole "survival of the fittest" goals but acknowledged that their methods were so different they'd probably would never be able to work together.
You know, before they got into a brawl.