Okay, stupid AU idea but hear me out (based off of that one comic issue about Zim putting Dib in an alternate reality where they’re brothers):
Basically some time after Dib was born/created and before Gaz was a thing and stuff, Membrane discovers Smeet Zim and raises him alongside his other children. And Zim basically grows up thinking he’s a human with a freaky skin condition and is still an asshole but is, like, nicer and stuff. But eventually Zim starts to wonder “hey, wait, I’m green and aliens are green and I wear a wig and contacts and stuff, this is weird, maybe I am an alien...”
And GAZ knew all along without anyone knowing, she just sorta accepted it and was like “really? Ya’ll didn’t know? Fucking moron”, while Dib, FRIGGING OBSESSIVE COMPULSIVE DIB, never actually considers Zim an alien in the AU because he has it in his had that all space aliens are evil and Zim is CLEARLY not evil - a lil’ SHIT sometimes, but DEFINITELY not evil. So yeah, a bunch dumb crap happens and eventually they’re like “shit, Zim IS an alien, wtf” and drama happens, I guess. X’D
This was a lot better in my head, I will admit, lol