I'll fully admit, the worldbuilding work I've done for the Banshee space-opera verse has was done in a strange way. Namely, I did a combination of top-down and bottom-up building.
To define these terms: top-down building is when you make first big decisions about the different cultures, borders and interactions, and then build the map to meet those needs. Bottom-up building is when you create the map first, and then figure out cultures, borders and interactions based on what the map demands.
Neither approach is right or wrong, good or bad. They're just different ways to go about it. In fact, I advocate the hybrid approach.
In this case, the "top down" involved the decisions about some of the alien cultures closest to Earth. I knew one thing I wanted was a large Alliance in close proximity to Earth, who had taken a preservationist/non-interference attitude to the planetbound cultures in their spheres of influence. I knew I wanted an aggressively expansionist culture (the Paxin) and an imperialist culture (the Surani), and a recent interstellar addition who would give the humans a good fight (the Krek'nik).
Also, in general, I wanted our interstellar region to be filled with intelligent life that was all, more or less, in the same place-- i.e. everyone had gotten into space or could potentially get into space within a few centuries of each other (or in the case of the three "old" powers in the region, a few millennia)-- which, in cosmic terms is the blink of an eye and highly improbably, unless you incorporate a serious don't-poke-this-too-hard conceit. Which I did.
But, in terms of "bottom up", I knew I wanted the stellar geography to be sensible. Real stars where they really are. Now, this meant I probably did a bit of homeworld-fudging-- I'm given to understand that Procyon is probably too young a star to have a planet with advanced life on it, for example-- but that fulfilled at least a sense of verisimilitude.
But the other "bottom up" aspect I had to ask myself was-- what else was out there? I had the raw data on stars within 150 light-years of Earth, and from that, crafted some randomization for each star: Are there planets? Where are the planets? Do any have life? Is that life intelligent? How technologically advanced is that intelligent life? Have they achieved FTL travel, and if so, when? From all that, I could build up exact details of the 147 starfaring cultures, and how their potential interaction might be.
This bottom-up method gave me the opportunity come up with ideas that I might never have had without star-map based data fueling it. Seven alien cultures in relative proximity to each other form a loose coalition. One advanced culture with no one in proximity builds a sizable empire before encountering any pushback. Another with a powerful aggressive species nearby builds their culture on defending themselves.
From this, I found more interesting discoveries. I devised a little equation based on expansion (how many colonies or outposts a culture had) and their tech level, and were able to calculate who the true "First Level" powers in the region were. And from that, I've been putting together how the Astronomical Geopolitics (Astropolitics?) really work.
I'd like to think doing that work-- while anal and time-consuming-- has created something a little more organic than just a top-down alone process would have.