Just a brief note -- any diplomacy that is added would be for a very small, third-party faction like the Zint aliens. I doubt that tech trading would be possible, or anything else too advanced, but rather just another strategic decision point. We'll see if even that happens -- you have some great points -- but I think that having a bit of diplomacy with that sort of smaller, rarer, third party AI might be cool (but definitely expansion-only). As for the main AIs, there never will be any diplomacy with them: they can't be bargained with, reasoned with, etc, and that won't change. And diplomacy with other players will also likely never happen, since we're all in this together as humans just trying to survive. The only question is if some form of diplomacy is used in interactions with a new, smaller, third party that might control a couple of planets in the galaxy.
Anyway, just thought I'd leave a brief note, I'll probably not get to any more continuation on this topic until I get back (but everyone else feel free to weigh in with your thoughts, this is an interesting topic and CautiousChaos has some great points).