I think this is a great idea - perhaps a modified minor race could do this. A few different minor race categories would be good -
1. actual new civilizations
2. outcasts and smuggler groups
3. Aggressive and expansionist minor races (Snathi, Dark Yor)

all with different AI algorithims so they behave in a different way. I think the super ability the Drengin have would be good for the smugglers (the one that give you free ships when war is declared) so they would be a bit tougher to take over too. Oh, and also make all of them unable to build construction modules on their ships, so they don't waste time spamming influence starbases everywhere that don't do anything or gobbling up all the resources and not have other civs be able to buy them. This is something I am hoping for in the next expansion.

Haha minor races are lacking? If you ever want to have some fun, give a minor race two or three worlds... They go nuts! It was explained a while back that a minor race wont really expand on its own. But something about them gaining additional worlds overrides something in the code and they start going for blood!

If I ever want to mess with an AI I give the minor race closest to them some planets and cash and get them to declare war on the major... Entertainment at its best!

Uh, last time I looked you could buy ships already.

But I do like the idea of independant corperations etc, just what to do with them ?

Actually there's a list of offers everyone ignores, and these offers are made by different companies.

True, but with those companies, you can buy only a ship created in your shipyard, which mean that you actually have the proper technologies.

My idea was to be able to buy a ship, let's say, with beam weapons even if you haven't researched beam tech at all (concentrating on missiles for example). Could be in a new screen like "Market Place" where there would be a list of corporations with their goods. The list would expand according to the degree of exploration of your Civ.

I'm with the Eradicator there.
You can't really go out and buy a aircraft carrier or submarine today. There isn't a free market for those things, that's all controlled by governments (mind you, I don't do that kind of business all day, so this is just a 'as far as I know').

So if you want to get some ships from a different branch of the techtree check out some other races. In my latest game the Paolos had some pretty good ships in stock (I was technologically behind because of the colonization rush). If you're good at diplomacy you can get them for a reasonable price and it sure makes for a surprise when you roll in a fleet rocket-frigates when the AI thought it'd beat you by defending well against your beams.
If you want a minor race to shell out ships make it declare war on someone on the other side of the galaxy. Works nicely.

How about adding that here:
Foreign Relations -> Minor Races -> Black market -> Speak to
They've got techs, they've got ships - and they have no conscience. They also maintain pirate ships and raid trade-ships once in a while, or even capture other race's military ships. You can trade with them and they'll like you and spare you. Or you just pay them tribute. Or you use espionage and gain knowledge of the location of their ships. Or you ask them to attack someone - costs money, but your target won't know who send them.

That actually puts both of your ideas into one that is even very senseful in GalCiv's context. It doesn't require many changes and is still very effective.

Hope you get the idea...

Interestingly I don't, actually.

I believe you are trying to make two points:
First, if handled carefully you can do everything the black market does, via diplomacy on the 'official' way. So instead of anonymously making pirates attack an enemy you don't want to wage war on yet you make some minor race declare war, pump them full of money, techs and ships and thus also get your military conflict. Or you buy ships you can't build yourself from someone else.
Secondly, there are already quite different way of gaining victory. We should be careful not to create a too wide band of strategies.

Let's assume I guessed right:
On the first one you really do have a point. I'm really considering designing new minor races to get that job done. I will have to dig in a few xml files to see how well that'll work. If that works a diplomacy screen for that black market isn't necessary. However, I still keep on to the ideas of pirates and domestic trade.
The second point is a bit more difficult to comment on. I can see that a wide array of strategies can be harmful to the game if you play a campaign that is designed to be rather short. It is difficult to switch strategies in a short time in the original GalCiv2 (which I modified for that purpose), so if the strategy-branch spreads too far the "Make it or Break it"-part of GalCiv is strenghtened. On a long-time campaign, huge galaxies, few enemies, few stars, few planets, few habitables, many possible strategies are important to keep the game interesting. Every new tool improves the gameplay a lot, because it gives me the ability to play out even more complex plans to domination.

By the way I don't plan to buy DA, because most of it's changes don't fit my playing style. I'd have to rip half the addon out so the gamemechanics are basically the same, just with some new stuff like that nice ship-review screen and some more ship-juwelry.