I have always wanted to do this in Sins or other 4X games. It annoyed me to no end that Sins had long trade routes you could raid but it was pointless because you gained next to nothing from doing it. At the very least if other players have unguarded trade routes you should be able to run in and steal everything that isn't nailed down for a serious economic reward, and a serious penalty for the robbed player. A way to disguise the controller of you ships would be icing on the cake.

Instead of pirate civilizations (which doesn't make sense to me), we should have some ships assigned to do pirate activities (privateers).

I'd like the idea of NPC pirates. After each skirmish, a number of destroyed ships are added to the pirates' forces instead of being removed from the map. A small pirate force could realistically grow rapidly if left unchecked. Complete pirate dominance over space would make for an interesting game.

Pirates would originate by rebelling from existing empires. They don't have planets, just ships. Their ethical alignment would always be opposite of the leaning of faction they defected from, or randomly chosen if the initial civ is leaning neutral.

You could negotiate or bribe pirates to carry out various tasks (attack fleet, attack ship, blockade X for Y number of weeks, etc). Pirates would only accept money, which would (invisibly) be used to cause more ships to rebel and join their cause.

Maybe a pirate DLC or Mod someday? If anyone else is interested, I could start a dedicated thread (I could also write a long post with exact suggested mechanics).

Lol, sorry about that. Didn't see your post. I'm thinking in terms of Civ IV where we had Privateers that we would use against enemy ships and the AI would never know who did it. 

I realy like the idea of a nomad civ

they can't build starbases or colonize planets but build the largest ships

so they build ships from ships...

They could have gigantic and heavy armed colony ships flying among solar systems

> this ships could have unlimited range and range of others smaller ships is calculated from them

They could deploy in empty solar systems and exploit planets

they could exploit inhostpitable planets so they could share a solar system with an ally with is permission??

they could recycle destroyed ships

they could specialize in :

• movement (speed + range (if limited) + speed/cost of deployment and undeloyement)
• piracy (black market + stealth...)
• farming planets and asteroid (to have a more classic gameplay)
• low costs ships and great recycling activities
• fight abilities with mercenaries diplomatic capabilities
• trade
• exploration (detectors, range...)
• ...

they have no influence but influence of others has no effect on them

It make me think about the Quarians in mass effect

This is where espionage comes in, using spies you should be able to determine which race the pirate ships belong to and then if they make an ill judged act of piracy against you you will have a very good reason to go to war against that faction, same in reverse if the AI has sufficient spies to know that your pirate ships belong to you and you attack them then they'll go to war with you too.

Also Any ship that is flagged as a pirate ship would be targetted for destruction by any other civ if the ship is found. So for this to work for player to have pirate ships, and the AI too, we'd probably need a cloaking technology

Then it would make even more sense to sponsor stealth pirate base to spawn the pirates ships safely away from your borders.

In fact there is a major problem with a nomad civ :

if they leave solar systems, others will colonize them and they won't be able to come back

at the end of the game there will be no more available solar systems (or the galaxy is enough big and impossible to colonize at 100%)

Nomads will just have poor systems with only inhospitable planets at the end

But this problem could be part of the gameplay nomads could spread widely at the beginning but then have to settle in the middle of the game

They have to spread to maximize their trade or pirate abilities 

if they choose the pirate way they could hide in inhospitable systems and move to an other when they are attacked the fact that the systems is poor is not a problem as their incomes come mainly from piracy (poor system ensure only a minimum income)

If they choose the trade way they could settle in more interesting systems but if they choose very interesting systems others will be tempt to military drive them out and they are weak... So they could first take the most interesting systems and then leave to other systems, less interesting, when other players came in the area it could be part of a negotiation.

Well, you can have the nomad civilization be able to enter populated territories and be able to extract percentages of materials from it (civilian/corp trade and such) and resultantly expand the host civ's economy or some such.

Allow the nomads to mooch off the host civ and in return the host civ will receive a bonus somewhere.

The nomad civ will start off having to build "mining" bases or some-such. As they move on they either completely disassemble them for the materials or leave them for another civ to find, which could result in a positive diplomatic outlook in the future.

As civs become more widespread and more powerful, the effect on the nomads would be increased, too.

This would keep them pretty much on a parity throughout game time.

May have to give the nomads an increase in diplomacy and economics and maybe increase system unhappiness within a certain range.

There would be pros and cons for the host civilization to consider.

How so?  I could easily see the humans with the Patriot fleet going nomad as they get pushed off of the colonies, kind of Battlestar Galactica esque.

They could still be able take resources from the uninhabitable planets if the civ does not use the its military to stop them.

I see two possible solutions to this problem.  First, destroy, or hopefully capture, enemy colony ships or take a page out of the Mongols and let the nomads create ships that would make habitable planets uninhabitable.

In the original, they were constantly running into inhabited worlds.

See some of the posts above.  There are perfectly viable ways for nomads to live in a crowded galaxy.  In fact, a crowded galaxy means there is more to loot.

Same would go for the human fleet in my scenario.

Make your case.  In a previous post I have provided several advantages a nomad civilization would have. 

Plus, I would not expect any race to be completely rational.  A nomadic lifestyle may seem more preferable to an alien race.  Historically that has been the case and, though much fewer in comparative numbers, even today there are still some cultures who prefer a nomadic lifestyle.