Here's the initial write-up of my setting, as (basically) jotted down in my notebook (I don't have a website, so I'd have to use either here or email). It's not fully thought out, in terms of aliens, but it covers most of this and parts of the two nearest Milky Way arms.
To get on with it:
The Interstellar Republic ruled over half a Galaxy, and had for 3,000 years. After 2,700 years, it was beginning to strain at the edges, but the strain was ignored until a private scouting venture discovered the human species - the scouts barely managed to hold onto their lives and property, and returned, frightened, to the Republic with the location of Earth. After a long debate, the Republic launced an expedition and formally contacted Earth, which launched the human species to the stars. After 100 years, humanity was considered integrated, but its internal tensions had never faded - and when dissident humans joined or conflicted with dissident Republican factions, civil war erupted. Another hundred years passed, and the once-tl20 Interstellar Republic was ash and ruin. On those planets which had been able to support life, many remnants subsisted at tl8, and more had reverted to tl0. Only a very few areas - those farthest from the war - had even retained tl12, but a few managed to keep tl15 - those on the very fringes of the other galactic arms. The last bastion of Republican culture, the Manische Consortium, had retained the highest level of tl15 available, and only because it was in a stellar cluster which was rich with resources, as well as hasty planning once the end was obvious. Now, 16 centuries have passed since the ruin of the Republic, and the Consortium is expanding again, sending a few major and numerous minor expeditions both out into the unexplored reaches of the Milky Way and back into the wasteland that was its ancestor in a hope to recontact whomever it can - though what will be found is entirely a mystery.
The tech level of the Consortium is a middle tl15, but it involves more advanced computer and biological technology than the OTU, and takes some hints from both SJ Games's (?) Transhuman Space and GURPS Biotech, such as sapient AIs and genemods. FTL travel is a version of hyperspace, with a maximum travel range of J12. Most star vessels can do about J8, and have much more efficient power plants. Sensors also have a range of ten light-seconds, with military models having more. On the whole, most technology is more interactive, with an emphasis on both ease of use and sophont (both biological and artificial) control.
