Culture should have an effect on diplomacy. A BIG effect. It should be as big as ships in terms of the game.
Think about it:
Planets should have multiple sets of alliegiances. If a player has allegiance>than owning player on another planet, he should be able 'force' consessions and gain bonuses based on a number of factors, such as number of planets, total amount of galaxy controlled, etc.
From time to time, the player could make demands like, "Give X crystal/credit/metal" there could be tech trading. The number of "concessions" or demands you get to make is proportional to the amount of culture you control.
Neutral planets should join planets that acquire them via culture. The militia should join you. Neutral planets should have 'weak' culture and be acquired quickly. Culture should spread through neutral territory VERY quickly.
Actually make it VIABLE to have a cultural build rather than just a military attack strategy. Investing all your resources into culture (and security/starbases) should just be as effective game wise as throwing all that money into buying assailants and a colonizer. Right now, a full fleet of combat ships is the most profitable investment one can make, since one can take neutral planets quickly, as well as harass the enemy.
Well, you know what? An advent player that rushes up the tech tree and spams culture stations should be able to grab 2-3 worlds in the first half hour, just as easily as the vasari player- this would actually be very interesting: a rush military player would be racing against time to prevent a quickly spreading culture web from acquiring lots of militia units, or eventually spidering out and eating his own planets. And enemy with 10,000 credits and resources invested in culture stations should be just as dangerous (Or admittedly, slightly less) then one with that much invested in ships.
Granted, this would be a really radical departure from the core gameplay, but it would make Sins of a Solar Empire more of an EMPIRE building game with multiple choices to victory, rather than just building the best and biggest fleets.
Borrowing from Rise of Legends sounds like a great idea. An economic build would actually be viable if you could buy a neutral planet and the accompanying militia.
Make attacking trade ships a bigger deal. Cutting off a line of enemy trade should significantly kill their economy, maybe inflicting like a 50% penalty on ALL trade, and improving yours slightly.
There should be consequences to fighting. Destroying civilian structures, ships, and bombarding planets should increase "ruthlessness" a similiar mechanic to BadBoy in Europa Universalis 3. Players with a high ruthlessness score have less loyal planets and are more vulnerable to cultural attacks. In all honesty, someone who plays as a 'ruthless' ruler should just be playing vanilla sins.
Players trying to 'play nice' should gain massive bonuses to culture and allegiance (Or start with them, which decrease as they fight) that allow them to pull off the economic and cultural strategies above (Neutral worlds are cheaper to buy if your ruthless score is low). Hiring pirates wouldn't affect your ruthlessness score at all!
Improve AI responsiveness to orders. Give them missions. If I ask them to goto a system and lets say, "Destroy all ships/structures" I should be able to pay them X amount based on the number of enemies in that system and watch them go. These missions could also be given to players, with the sum payed contingent based on custom set timers.
This is a big one. All 4x games have tech trading, except sins! There are many ways to do this:
Allow trading of ships, planets,structures
"Loan" the use of research facilities to an ally (If you have 8 labs, and give him 3 for 15 minutes, you are left with 5 labs)
Finally, trading of techs (Perhaps limited to the numberof structures owned, based on fleet cap, based on time, etc.)
By spending money on spies perhaps you could:
Steal an enemy tech (from a similiar race)
Take over a number of enemy ships (Restricted by type/model) at an inoppurtune time
Cause pirates or 'revolutionaries' to attack
Takeover/Blow up troublesome structures (Superweapons, Starbases)
Freeze planetary income/production
Disable enemy abilities in a gravwell.
It could work like galciv. You have a pool of spies, and devote them to certain gravwells. Overtime, random happy things would occur or you might gain a pool of 'powers' to use. Not sure if the engine supports that though..
All these changes are probably fundamental core gameplay changes...for a micro expansion, maybe too big of a change. It'd be nice though, and certainly worth my 10$.