I both Zim/Gaz and Zim/Dib (and Dib/Tak, Zim/Tak, Gaz, Tak etc.) and while I actually like Zim/Gaz more than ZADR in this case the fujoshits might have gotten it right in the sense that that relationship does have the most to work with. Why? Just because Zim and Dib are co-protagonists and most of the show is about their interactions.
Also the unaired episode "Mopiness of Doom," if you read the script or listen to the recording, would read like a love story in other contexts, probably on purpose for humor. Basically, Dib decides to give up on chasing Zim for "real science. Zim gets severely depressed without Dib and mopes around eating so much food he gets fat and lies around all day. Meanwhile, Dib pretends to be happy but is also miserable. In the end, Dib goes back to chasing Zim because he just can't stay away and the two are happy again. That's way more to base shit on then a lot of ships.
That said, it doesn't fucking matter what ship is likely to happen. You are talking about the love lives or make believe cartoon characters. All ships have the same percentage chance of "really" happening: 0%, because the participants don't exist. Shipping is autistic anyway so once you do it, just ship what you find fun and entertaining just don't try to bother the showrunners about it and get mad when they don't put it in the show that is fucking retarded. This kind of "it has to be in the show let me bother the people involved or harass fans about it" psychosis is when shipping becomes cancerous, not fucking around for fun.