1. be able to make requests of the AI for money, resources, targets, designate a fleet rally point when playing SP.
2. be able to gift ships to allies, or at least surrender their control to an ally fleet.
2a. vasari allies can use vasari phase stabilizers.
2b. a "kostura lite" with roughly the same research requirements as the phase stabilizer, which only opens a phase gate, no damage, no ability disruption, etc.
3. be able to evacuate a world so it can be colonized by an ally/teammate, or maybe build structures in their gravity wells.
4. More degrees of control over ship vision, ie what certain ship classes see, what sort of info in the grav well you share.
5. Time increment, or percentage of income, payments of resources to allies.
6. Some method for performing a decent backstab, what's diplomacy without a good backstab?
7. An option to make another player a vassal by dominating them somehow, obligated to pay tribute in some form, well, obligated until they decide to rebel.
8. Converting neutral, even formerly colonized, worlds to your side by overwhelming them with culture.
9. Culture/trade weapons, ie trade ships loaded with explosives which infiltrate and blow enemy trade ports, a ship that bum rushes a planet with "informative leaflets" and hankers the culture, **Edit** or trade goods tainted with a virus
9a. Antitrade/mineral ship weaponry. Interrupting these things is a powerful tool. I mean, I guess you could jury-rig flak to do it as it is now, but maybe something more elegant.
9b. A way to steal the cargo of trade/mineral ships for your own benefit. Ie, raider ships.
10. Spies which can gather intel about your diplomatic settings with other players, steal limited ship and planet vision, siphon income, delay production, with advanced abilities to detonate structures or instigate revolts.
11. Counterspies which root out spies in your empire and infiltrate and hamper the spies in other empires.
12. Pirate fleet force rework to keep them more menacing throughout the game, makes paying them a better option.
13. Antibounty, pay the pirates not to attack a certain target.
14. Specific missions to the pirates, ie 5k to raze a certain planet.
15. Neutral forces which scale as the game goes on. ie, they add a couple more ships to their fleet, maybe build a mineral extractor.
16. Ambassador/Embassy ships/structures as a means of channeling/directing diplomatic options between players, staging spies, gathering intel, upgrades that lengthen the time to break treaties, etc.
17. Tech to allow you mask/inflate your apparent fleet/empire size from remote viewing/ship vision.