My first sci fi RPG was Star Frontiers. I think I got it in 1983. Having been profoundly changed by Star Wars and then Battlestar Galactica, I didn't quite connect with the 1950s and 60s influence, but we still played the crap out of it. All the modules, the knight hawks expansion, lots of made-up stuff on top. I didn't discover Traveller, on the other hand, until many years later, with plenty of additional sci fi games in between, like ICE's Space Master (Charts in spaaaaaace!) and D6 Star Wars, and some other odds and ends like Shatterzone.
There's still a huge fan community keeping Star Frontiers alive. You can download pdfs of pretty much everything that was ever published. I guess Hasbro treats it like abandonware? In any case, it's all there.
The rules are a little weird. I won't get into them. I had more than a few frustrations, let's leave it at that.
But something I think is cool, years later, is the premise that these four species discovered each other, and everyone got along, mostly. It's kind of Roddenberryish. When the evil menace of the Sathar attacked, then they banded together even more so.
Anyway, the point of all this is that I've been thinking about a Cepheus or 2d6-SRD setting based on that sort of premise. Like Star Trek using Traveller tropes. There'd be 4-5 core members of the Federation, some outlying worlds that are "integrating", and a few protected low-tech worlds that are red zoned.
Each of the 4/5 member species would have areas of expansion, with a few co-habitated worlds where they meet up, trade centers, crossroads, etc. Core worlds, colonies, and a frontier zone. Then some unexplored worlds out beyond that. Possibly some sort of aggressive enemy empire out there too.
I think this setup allows for most SRD stuff to be used unmodified. Instead of an Imperium, you have the Federation, with their joint military and scout bases and such. Smaller ship universe than Trek or SF (battleships in SF are about 300K dtons). Will probably cap things out at CE's 5000 dtons, with probably only a few of those.