I have to say, I really like the pact system in Diplomacy.  It really makes buddying up with an AI (yes, I play SP instead of MP, I have my reasons) yield some remarkable benefits, but something seemed missing.

One of the most prominent things that makes me consider ganking someone else's planet is Artifacts.  They're rare, they're powerful, and they show up ever so conveniently on the map.  They're just begging to be fought over...

So I had a thought... perhaps there should be a non-race specific level 8 pact that allows artifacts to be shared between players?  It might require a bit of tweaking to the wording of some of them (Manifest Dominion comes to mind) but it seemed to me that would represent the ultimate level of cooperation between empires.

It didn't seem immediately obvious that it would be particularly unbalance play, since it isn't giving any given empire abilities it wouldn't normally be possible for it to have... it just shares the wealth around.

Anyway, that's my big suggestion for Diplomancy.   Let me know what you think!