Much of why I liked the diplomacy system in Stars! There wasn't one.
It would have been massively complicated against the AI, far more-so than most games in this genre.
Against other humans, and MP was where the game really shone, it was perfect. Because diplomacy and agreements were made through negotiations and messaging, not arbitrarily bound to a system. Non-abstracted diplomacy at its finest.
But in a game where you're expected to interact with the AI on some vaguely meaningful level, yeah. Things at some point have to be abstracted. Whether it's cards (abstract diplomacy: SR2), options presented (hopefully with the AI having a "reasonable but wanting to win" and not "chaos-neutral" attitude towards things, even though the player will: Civ/MoO series), buttering up and alliances (Paradox style: EUIV/CK2) or even just the other participants acting "in character" (some people are for/against some things: Alpha Centauri), there has to be abstraction at some point.
Stars! worked well against other humans, because there was no framework to be burdened by. It didn't even bother with the AI. You were at war with them, and that was that.
At least you both knew where you stood diplomatically. AI is damn hard to program, especially the diplomacy and "memory of past actions" part, even more-so if massive wars are meant to be forgiven for the sake of peace and diplomacy sometimes.