Other rewards besides just resources might be nice. If I'm feeling generous to a smaller ally and he pulls off a tough mission for me, I might be willing to give him control of a planet that I'm just using as a trade-port cow on the fringe. Or maybe my buddy has a much better economy, so I'll give him a planet on my border so he can pack it full of defenses (as I mentioned in another thread, even at 250 trade rate, that's not a whole lot since it gets taxed).
Let me give them SHIPS!!!
Granted, whatever they take can't exceed their supply cap, and they might have to be same-faction, but if I have the resources and production set up to make twenty cruisers in a hurry to defend his planet, why can't I just give him those cruisers once they're made?
Ships and planets would be a much less used reward. Possibly make a credit transfer tax (100 per ship, 500 per cap, 2000 per planet) to prevent abuse. But still, team games could get much more interesting. One player could focus on dropping resources into capitals while the other cranks out frigates and cruisers and they just exchange owners.
I second the cultural influence victories. I think it's crap that my culture causes a rebellion that attacks me as well. Possibly, when a planet starts getting toward "rebelling", some computer-controlled loyalist patrol craft come out at first (hostile according to that empire's treaty status, but not under direct control and certainly no threat to a fleet) followed by rebel craft that actually are friendly to the cultures of the dominant empire and its allies. After some time, neutral territories should petition to join the empire with the culture dominance. This might lead to a neat strategy where you could seed an unoccupied star system by colonizing one planet, setting up a broadcast center, and letting your culture spread.
Also, PEACEFUL COLONIZATIONS! I shouldn't have to go smash the militia of each planet and then colonize by force. Militias shouldn't rush to kill me either. If I enter the star system with just a diplomatic envoy, then independent planets may or may not (at random or determined by some controllable factor) wish to join my empire for a price or special condition. I could buy them with a resource cost, or maybe they just don't want to be colonized at all and I need to bring out the guns. Maybe an independent planet, as a balancer, doesn't want to join the huge empire, so it offers to stand with a smaller one. Militias of planets acquired peacefully could stay on to act as patrol craft (and work with your fleet if it comes to defend the planet) or even join your fleet full-time, supply cap permitting. Make it an option of militia ships, if the engine allows. Make the militia controllable but not count against your cap as long as you're in that militia's grav-well. If you're willing to provide supply for them and accept the hit on your cap (and possibly a small credit hit to outfit them with all the enhancements you've researched for your fleet), then they'll fully integrate into your fleet and become part of your regular forces. Terran and desert planets would become much more valuble, as they could net you heavy cruisers very early on if you peacefully colonize instead of steamroll through. Also, possibly make the "militia conversion" something you have to reasearch as part of your fleet tree. Basically, you're reasearching a training program that helps integrate militias into regular forces. There could be other uses for the fleet tree with this as an option, though I definitely think that the "cap-ship experience" upgrades should fall under the fleet tree and not the civilian tree.
The benefits to peaceful colonization is that peaceful colonizations should come with free structures and planet upgrades. If I peacefully get a planet that already has a trade port, I should keep it. Or maybe, as terms of joining my empire, the planet eats the cost of building a trade port for me. Or four. I'm just tossing out ideas to make peaceful colonizations worthwhile. A peaceful colonization should start out with at least the population upgrade that makes the tax-rate even, if not in the green.
Not all uncolonized planets should stand alone! (there should be some non-expansionist enclaves sometimes) Not all uncolonized planets should be empty barrens! Honestly, why are there militias around empty planets? Make some of these places surrounded by non-aligned trade ports and refineries. Not every planet outside of an empire is population 1. Maybe make a few planets that you'd BENEFIT by letting them stay uncolonized. As in, they could provide you much higher resources or a few unique ships every few minutes.
The ability to trade the artifacts! If I've got a bunch of research done and all the research I've left is enhancement but not mission-critical, why can't I trade it with an ally who might have the jump drive relic? Just make us pick a planet to locate the relic, and maybe make us defend that ship if it's going on a long trip (but make it controllable! Let me pick its route!).
Less of a diplomacy thing, but a possible idea...let me designate no travel gravity wells for civilian ships. I so tire of my trade ships flying thru enemy territory just because it's the route. Either this, or make the civilian ships smart enough to avoid enemy territory and only pick routes thru known neutral or good grav wells (if they know an enemy starbase is there, don't let them go there!).
Please overhaul piracy in a big way! Just as I should be able to give specific objectives to a faction (as in Go to this planet an destroy all their weapons labs), I should be able to give the same to pirates. They shouldn't just launch one massive fleet at a time and go smash a planet to bits or die trying (sadly, Pirates can be more of a threat than the early AI, because an outnumbered AI wouldn't stand and fight). The fleets need to be smaller and faster (tradeoff for not having shields, maybe?) and focused more on stealing your resources or maybe an artifact on that planet. They should zip in, latch on to a trade port, and divert all that planet's income away from you. And if they really hate you, they'll do this to three or four planets at once.
Pirates should be more on a for-hire basis. Like negotiating with other nations, they will create raids at your behest for the right incentive. Of course, once your empire reaches a certain size compared to the number of independent planets left or compared to the remaining empires, pirates will either make their bill much higher or even flat-out refuse to deal with you, as they realize you'll shut them down once you're the only remaining power in the system. Pirate raids acting on your commission should not be considered hostile to your empire. Maybe not so much to your allies.
Pirates should launch small raids on unoccupied worlds! Another reason a planet might join your empire...if you agree to protect them from an impending pirate raid (of course, this pirate raid has a specific objective in mind that you must prevent, such as not letting x number of trade ships be destroyed or something similar and acheivable). And planets that get raided continuously and you fail to offer timely protection will rebel (if a neutral territory takes a culture structure, neutral culture starts spreading, possibly spawning a rebel enclave in your empire) or offer to join another empire that can protect them better.
Must pirates always be located one phase jump from a star in random maps? Hell, in most maps? They don't go between systems, so can we move them away?
Also forgot: second the needing to meet before diplomatic ties can be established. You should get a diplomatic ship that has to visit an empire to establish relations. Upon first contact, you CAN attack an enemy ship, but your ships shouldn't attack by default. Attacking a diplomatic ship is pretty much a universal declaration of war.
I think the jointly-owned planets might be a neat idea. Currently, the closest you get is your starbase in an allied system. But it would be awesome to have a "diplomatic seat" planet. This planet divides its resource income evenly between its owners, and each empire with a stake in it gets four logistical and defense slots to start and eight total, allowing you to exceed planetary caps of single ownership planets. Since building and defense of a joint-ownership planet requires everyone pitch in, buildings in joint sectors have unique functions to make them more valuble diplomatically: the owning empire can dedicate production to another empire. Toss two trade ports down and set them to give income to your buddy who's low on credits. Or maybe install a frigate factory that will let you build ships but it adds on to his fleet cap. Different-faction allies issues will have to be worked on, but it's a consideration. Maybe for this one, since you're allies, your factories get the schematics and can produce that faction's ships for them? Jointly owned labs don't give you access to each other's research (or maybe it does, at least the common stuff like wormhole and system jumps), but it does give you lab space, so labs on joint-worlds count for the lab count on both sides (even different faction allies), meaning 2 allies only need 5 labs a piece.
Artifacts found on joint-worlds are shared by both empires. Should no artifact be located there, an empire may bring an artifact to a joint world and share it with his allies.
The only world that cannot be joint-ownership is your capital planet. That should stay unique to you.
Declaring war causes chaos at joint worlds. An idea is to make another timer after the "declare war" timer. For 5 minutes after the "declare war" timer ends, any fleet can go and contest the location. Possibly sending in a colonizer to take the planet or seige machines to bomb off the new enemy empire could lead to fleets contesting these planets hotly, especially if an artifact had been located there. Imagine three or four fleets all vying for one planet? If no fleets show in the 5 minute window, then the planet will join the empire with the cuture advantage. If no culture advantage exists, the planet goes independent. If you have a planet with more than two owners, then the empires declaring war on the alliance will be automatically ejected from the ownership at the end of the five minutes (unless of course that empire wants to fight for it).
All just ideas. Amazing what I think of at this time of day.
2nd edit (yeah, I have ideas tonight!):
Pirates could perform more black market functions. They don't go around discovering artifacts or researching science stuff, but if they raid a planet with that artifact or a lab with a research topic, it should go up on the black market. Say...10% discount and no research time needed for the first paying empire that doesn't already have the tech. Artifacts are massively expensive (and price adjusted according to how big your empire is and how much the pirates fear you. If you own over half the system, they'll gouge you for 50K or more credits if they sell at all. But they're willing to sell to the guy with a six-planet cluster for 30K credits)...
Speaking of which, mayhaps I could sell research to allied factions if it's a similar research branch? I could set the price (probably 10% more expensive by default) and get compensated for my time and resources spent researching while he defends me. He pays more for risking less and taking less time, I get money for taking a risk and time.