I believe what I'm going to suggest has already been brought up multiple times over the course of these forums' life time but since Diplomacy makes for an excellent time to reconsider it I'm posting here.
I believe it is a mistake to have unlimited quantities of crystal and metal available available at the Black Market. I also think the in fact quite low ceiling (~600 credits) on price fluctuation puts too much limit on what could otherwise be an interesting game aspect: Economic warfare, using the forces of supply & demand to battle the enemy. This is something that's already featured in the game but simply not taken far enough.
If the Black Market ressources were limited coupled with price fluctuations being bigger, that would allow for some very interesting scenarios that would add a whole new layer of strategic considerations to the game. Example: In a 2v2 match, one side has managed to secure a large cache of ice planets, providing crystal. They also control some volcano planets. The other side, however, has mostly terran and volcano planets. They only hold one crystal planet. This isn't enough to fuel their war machine and they are forced to buy more on the black market. But because the side controlling the crystal won't sell and the quantity from the black market is limited, the price is sky-rocketting and they find themselves paying some 2000 credits per 100 crystal - and that's only if some is available!
Basically, by limiting the black market in this way players would be requred to think a lot more carefully about what planets they expand their empire to, as they want to secure both crystal & metal ressources. It would offer a lot of intriguing new situations, many of which could add to the diplomacy metagame. For example, there could be a 'ressource trade agreement' option (just like the current trade ship one) that would bind both players to exchange a certain percentage of a ressource to the other (metal for crystal & vice versa) for a duration of ie. 20 minutes. Or, a player with a stronger military might demand a tribute of a percentage of another player's income in exchange for forced peace for the next ten minutes.
I think this suggestion has multiple merits: It shouldn't be too hard to implement since much is already featured in the game and would just need a bit of tweaking. It also fits well with the whole theme of Diplomacy. Third, it simply expands the economic warfare aspect of the game enough for it to be considered a whole new feature. Fourth, it helps correct some balance problems with the current game by making ressource extractors more attractive compared to trade ports. Finally, it helps to deepen the ressource system, making credit, crystal and metal income less generic and instead something you have to give strategic consideration.
