Update: I kind of ship KaZaDr now. Dumping my thoughts about how to make it work/how I think the dynamics would kinda shake out under the cut. Writing those posts about Keef earlier made it ‘click’ in a way seeing fanart didn’t.
I'm trying to figure out how it'd get from the 'god this guy is annoying' point to 'wait I kinda get him' to the 'shit, shit, feelings?????' point. I feel like any getting-together scenario in a canon-ish environment would probs start with Zim and Dib already being together and purposefully trying to dick with Keef who allows himself to get strung along because he's just happy to be a part of things before they start feeling bad for him.
Like.... these two are assholes. They love each other but they're kinda jerky to everybody else, especially someone they have mutual distaste for. Keef's desire to chase after them would probably have him go along with anything if he got to get closer in the process. More than likely it just ends up in things being stretched to a breaking point as they see how far he’ll go to please them because it’s funny, and Keef is eventually angry he's being taken completely for granted because he sees how they treat each OTHER and it's frustrating. (They're still kinda jerks to each other, but it's in an affectionate way, and he doesn’t get what he’s doing wrong to not get to be part of that.)
They realize they actually do miss him when he storms off and have to admit they messed up, and when they make up they get the chance to actually all talk and it's kind of a relief to clear the air, and things go a little smoother after that.
They take him out for coffee. It is really awkward, since neither Zim or Dib is used to apologizing. Keef's a forgiving guy, though, especially with them, and when he starts opening up (which he does really, really easily, he thinks it'll get them to like him if he just tells the truth) Dib feels bad about pushing someone who was just desperate for any kind of attention- he knows how that feels, and although Zim won’t admit it, he does too. Zim thinks that being alone is normal and it’s kind of weak to want affection, that's how training goes, but Dib smacks him and mumbles out an apology. After that, Keef shows up more again. Gir got really attached to him, so not having him bawl that Keef is gone is good. It's actually kind of nice having an upbeat presence around that ISN'T Gir.
Trying to figure out how each dynamic works... I obviously know how Zadr does, but Kadr and Kazr? (Is that it? Is it Zakr? Gah, dumb shipping system.) Hmm. Dib/Keef is obviously a sunshine/grumpy dynamic, but they could understand how the other is outcast from a specifically human perspective. Keef could be the attentive ear Dib needs, while Dib could give him attention back that he needs too. Dib also has control/possessiveness issues, so honestly, when all of them get together it's just clingy x100. I don't think Keef would really mind as long as they like him- he's getting positive interactions now, and goodness knows he has boundary issues of his own. Having someone around to be positive would be great for Dib.
Zim and Keef... There could be the 'positive attention/validation' thing here too, but I think he'd benefit from someone who's just genuinely cheering for him. Dib's sort of sardonic even though he admires Zim, so having unfiltered praise, while boosting his ego, could be something he didn't realize he had to always make up for himself until he got it freely. Keef, meanwhile, idolizes Zim- getting Zim clingy and possessive/affectionate to him is really all he'd want, going from no one to two people who would now Die For Him. It's a lot, but a good sort of ‘a lot’.