I haven't played DW, so can't really comment on similarities there. But I imagine that both they and we are pulling from similar general tropes in sci-fi overall, honestly, heh.
In terms of how the races behave with one another in one game versus another, yes that absolutely does vary based on situations. Sometimes the "klingons" cower in terror from the "ewoks," and I always find that hilarious when it does happen. That said, even then there is a very distinct feel of dealing with klingons versus ewoks when you personally interact with them -- but their attitudes towards you and toward one another can really change the feel of how the overall atmosphere is. Dealing with a burlust warlord is always very different from dealing with a peltian collective, but dealing with a warmongering versus a meek collective, or an ascendant warlord versus a desperate one are four very different scenarios.
One of the funny/crazy examples of emergent stuff happening lately is the pirates are sometimes setting up kind of a federation of their own, heh. They form a pirate empire and then get so strong that they suppress all the planetary fleets of the other races, thus enforcing peace through violence (and preventing the actual federation from forming). That was... unexpected. It's happening too frequently right now, but it's cool to see that happen sometimes.
There have also been some cases of where a race was super-compatible with a planet with low land area, and they then just absolutely sprinted into space aggressively and started taking over everything incredibly quickly. Basically a zerg rush, out of the blue, which also was unexpected.
And then there's a whole variety of humorous tales of woe relating to events and so forth and so on, but those are more in the range of the "expected randomness" I suppose you could call it. It will surprise the players quite often and in fun ways, but it doesn't surprise us as the developers since we coded those to sometimes come up. But even with those, the combinations of those can lead to some pretty unexpected end results since there can be kind of causal chain reactions.
I'll be doing a much bigger writeup on the whole butterfly effect stuff sometime soon, but right now haven't had time. But those are a couple of very broad examples.
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Teal -- very much agree on what is "cool" when something is real versus just a game, heh.
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In terms of the federation, they can "leave" if they die, which is possible. Right now that's the only way that they can leave, although at some point likely we'd add some ways they could leave.
Once in the federation, a race can still personally hate your guts, and may wind up sending assassins after you, etc. They may even kill you (losing you the game) despite being in the federation! And even if their official government is not sending assassins after you, the race may have a ton of citizen unrest leading to AFA demonstrators and insurgents, which then wind up fighting with you and possibly killing you.
On that note, it is possible to get a race into the federation even if they hate you and you have like the worst relationship with them ever. But to do that, you have to make sure that their relationship with some other race is really good, and then kind of backdoor them into the federation through a political deal with that other race. Those sorts of deals can only be brokered by some races, like the Skylaxians. So if the Skylaxians are dead and the Thoraxians perma-hate you, then you may find yourself in a situation where the only way to win the game is to kill the Thoraxians. Just as one example. You haven't lost the game, but you have lost a lot of options by then; you are highly unlikely to be able to remotely kill the Thoraxians yourself, so you need to set about warmongering the other races (in the federation or not) to go kill them. And then help a bit as you can.