Diplomacy features?  Hmmm, I'd rather see a new package w/ significant improvement ot economic issues.  But if diplomacy is what you want, consider these:

Joint Defense Force: Teams participating in a Joint Defense Force contribute ships, perhap in some sort of value ratio.  JDF can only be used for defense.  Whether the JDF is player controlled, AI controlled, or both is up to you.  I think AI would be better as you could make it more team-neutral in it's choices of movement.

Purchase of ships: Let one team purchase (or offer to sell) ships from/to another team.  Qualifying conditions based on diplomatic relations.

Joint exploration: Multiple teams finance joint exploration (or spying, using very expensive "cloaked" spyships) w/ both geting access to found info.

Delayed Interstelar communications: Assume communications cannot go thru wormholes... use courier ships to carry offers & replies.  It adds a delay plus a risk of the courier ship getting killed somewhere along the way.

Embasies: Make plant vision depend on the presence of an embasy, which in turn depends on the arrival of the ambassador's ship. 

Commando Raids: Link sabotage, insurection, coup attempts, and run of the mill spying to the arrival of a commando ship, discovery of which would be very bad for intra-alliance use.

I like this idea... neutral pirates... mercenaries???  No wait... not pirates, but Privateers!!! 

I also really like the intelligance ideas floated, espionage especially.  i would love to have either a spy network, or maybe some kind of stealth intel gathering ships... and of course some ways of countering or detecting them as well.

I've also thought it somewhat odd that all 3 races use the same type of currency, i'e. Credits.  It seems odd to me that all 3 races, especially the vasari would all be using "Credits". I would think they'd be totally uninterested in TEC $$$, so the "missions" seem sort of lame.  If you ask me, the only way to make friends with the Vasari would be to give them one of your worlds...hehehe.  Ready to give up that asteroid for a peace treaty???

Upgrade each planet with their own form of leadership? Communism, Facism, Capitalism, Libertarianism, etc etc. These should provide bonuses for the planet, for example:

Facism- Cheaper ships/orbital structures, lower population

Communism- More Civillian slots but less Defense slots

Libertarianism- Higher population but lower allegiance

Capitalism- Higher planetary income but higher ship/orbital structure costs

Khezef and Dargoon999 nice ideas karma up.

Well, since you asked, my idea was something along the lines of 750$ + 225m +75c for the initial upgrade, but having the option to change it should be available but at a higher cost; 1500$ + 450m + 150c for each upgrade after the initial one.

As for the UCGW's, the standard rate should apply; they are not part of a specific planet's gravity well.

EDIT: But the refinery itself could be located in a gravity well with the said upgrade... mmm... I dont know, I'll leave that one to you.

Military, economic, industrial, and research "focuses" could be 4 seperate sliders... as you slide the silders, say... from 100%,100%,100%,100%... to say... 150%,50%, 100%,100% you would get... (they allways have to add up to 400%... and for balance issues... prolly should not be able to go above 200%)...

(now, i am picking things preety much out of the air here, just like my other "government sliders" idea on page 7)

...bonuses and penelties... say... you get 25% more tactacal slots, and 25% faster ship build speed, and 25% cheaper emegerency facilities, but you would lose 25% of your logistical slots, and 25% of your trade port and tax income, and 25% higher cost on increasing logistical slots and population slots.

or...

if you changed it to 100%,100%,150%,50%, you would get an extra 25% extractor income, 25% refinary income, and 25% more construction frigs... 25% cheaper tactical slots, but you would lose 25% effectiveness on any research labs you had built in the system.

(in other words... if you had 2 civic labs on your homeworld, and you lower research focus to 50%... now you only have 1.5 civic research labs, and say goodby to researching ice and lava planet colonization if you havent researched it allready, and things would take 25% longer to research, and civic technology would cost 25% more. (these would be, ofcourse an average over all the research focues present on labs... inother worlds... 2 military labs on a 50% world, and 2 military labs on a 150% world would balance eachother out... and your military research would act just like it does now))

say... just totally random numbers here... but lets pretend 0% focuse means 50% normal whatever, and 200% means 150% normal whatever...so 0%, 200%, 200%, 0%, would mean that you got 1/2 ship build speed, 1/2 tactical slots,50% pricer emegerency facilities, 1/2 effective research facilities, and 50% increases on logistical slots, trade, refinary, and tax income, and all that other good stuff.  

and just like the government slider idea... you would set what you would like it to be at, and as time passes the focuses slowly change over time to reflect what you want... but you can never set your sliders to settings that would... say... cause you to have not enough slots for your current buildings... inother words... you cant set your economic up to 200%, build a zillion trade depos, and put it back down to 100%, keeping all the builings you built.

Remember... just like all my stupid ideas... numbers should get a rough approximation of whatever... and are just for the idea... there are many balance issues that have to be addressed, and many possible bonues, and penelties that i did not address...

The important thing is this: My 2 ideas in this thread are just an attempt to make something that is "on" or "off"... like your either econ focus, OR researh focus OR military OR,... or your a democracy OR a commune OR a dictatorship... and make it something.... smoother... with "shades of grey" inbetween.

Short and simple: I think the game could use an "area-of-effect" attack cruiser with some form of an uncapped damage attack.  It would outrange repulsion (thus acting as a counter to the battleball) and be less effective against mobile units like light frigates.  It would make it harder for LRF and flaks to stay in a tight group and cover each other from fighters and light frigates respectively.  It would also help capital ships scale better into the late game, since huge swarms of units (and thus their focus fire) would have to be used more cautiously, whereas a few big units wouldn't be nearly as vulnerable to this.

That's my 2-cents.  If there is only one new unit (per faction) in the next expansion pack, I'd like to see it be an area of effect cruiser.

This is another instance where having independent trade ships which can be set routes by the player might work.  Currently the player doesn't interact with trade ships and their cargo is notional, also the trade port itself is so much more valuable that destroying trade ships in a gravwell is almost a waste of time.  If the player had to build trade ships independently of trade ports then they could be ordered to routes of greater or lesser value, formed into fleets to make convoys, captured by other factions or used to exploit the trade routes opened by another allied faction- to the detriment of that ally's own trade....  Also, worlds with several trade ports would be avoided, unless there were huge queues there, and a very simple system of rules for trade- like with unlike, route length, decreasing value by transaction frequency- might result in far deeper gameplay.  The current trade route method is more than slightly crude..

The problem with feed isn't resource transfer in itself but instant feeding, which leads to rush tactics being tremendously more effective, even with the penalty on resource transfers.  A simple solution without upgrading trade would be to put a delay on feed, with the resources being lost to one player for a while before they appeared in the other player's account.  However, by far the better solution is to require that sufficient trade ships, taken off their trade routes, carry transferred supplies to another player's trade port, so that valuable convoys come into existence.  That would stop very early feed- and make the game more complex and enjoyable.

No graphic upgrades at all would be required for this, and trade is very core to 4x... and having a trade upgrade would mesh well with the new pirates and diplomacy..?

interesting ideas here. I like the more customised trade and having to buy trade ships. of course, either the trade port would have to be cheaper then or the total return greater, otherwise trade would be less interesting. here is how I would like to see it: 1) you don't build individual trade ships, but small fleets, a bit like in MoO 2. these fleets act as one unit, think of strike craft squads. that way, you don't have a constant stream of ships going through, but rather a few small fleetlets. seems a bit more realistic and would easier to attack too I'd figure. 2) each trade port only supports a certain number of those trade fleets, like 2 or three of them. so, if you want more, you need more trade ports.

I'm not sure yet about designating your own routes. tbh, I though this aspect at least was ok to be automated and I don't quite know what the purpose would be, if you had trade ports at most planets anyway. ships would go anywhere to everywhere anyway. it would only make sense, if trade ports themselves were a much rarer phenomenon. which is unrealistic, since they are the primary filler of available logistics slots, after immediate need of labs and shipyards is filled.

on the planet development side, I did have an idea also. it could orient more conceptually to the way starbases work. say, every planet has a limit of total development upgrades it can use, say 12 or 16 or so. should vary from type to type, roids have less, desert planets have more. but aside from that, you have more liberty in how you use those slots, there is no 'max. 3 pop upgrades' for a planet. you get as many as the slots allow, but you have to live with shortcomings elsewhere. just a thought of mine.

Some kind of new and better diplomacy screen with more options to trade (think civ IV or Galciv II)

NEW PLANETS   (of course)

A story driven campaign!!! I want to know the story!

the rest is already said.

There are way too many posts for me to bother reading but I gotta ask;  does anyone really care about the diplomacy aspect of this game?  I usually just lock the teams at the start anyway.  It just seems like such a waste of manpower to work on this aspect of the game.

I'll probably still buy the expansion because the third expansion will probably require it but I kinda feel like it'll be a waste of money since this direction doesn't interest me in the least.

Bottom line I just hope there will be other content that will keep players like me from feeling exploited.  A new cap for each race and a few ships or structures would be enough for me if this expansion keeps to the reasonable price tag of 10$.

One of the things I would most like to see happen in Sins of a SOlar Empire is the elimination of phase paths os the only form of travel. Rather, Phase paths should serve as trade routes, and phase expressways. It should not be necessary that ships travel on phase paths, but phase paths should give a huge bonus to speed.

I like the Idea of moons too, planets should have colonizable moons that serve as super-mining centers. These moons would provide large amounts, of minerals and crystals, but asteroids should still serve as the back-bone of the mining industry.

I also second everything Icen9ne, Annatar11, and wnmnkh said.

edit:

oh yeah, and a type of campaign mode would be nice (but for marketing purposes I would save that for a later more expensive expansion).

edit(2):

THe player should also have more control over trade, and player-to-player trade should not be instant, instead the player should send caravans containing minerals/credits/crystals to foreign empires. These trade caravans would have to be gaurded from pirates, and other Empires that might cease the goods.

Sharing of tech (Armor/Shield among all, weapons among a race that can share them), perhaps even compounding research among allies. Also, as mentioned above, sharing of Phase Stabilizer Nodes.

Sharing of culture (Abilities will affect allies, perhaps even adding/compounding, to a cap. No good having 99% mitigation due to advent culture... perhaps only +25% per ally you have, so 8 would become 10, then 12, then 14...)

If doing trade... have the trade calculation extended to include all allies with trade centers (in a blue line instead of a white one, attached to the ends of your own best trade route)

Heavy Constructors have a construction ability to help out planets (that is better than the planetary constructor ability -> a bit faster). So you can jump in a couple of them to help out a freshly-colonized planet get up defenses etc... can also be used to help out allies.