Early Vilani: Campaign
An alternative to the early 3rd Imperium idea is to take the time back even further to the early Vilani expanding out of Vland.
Say the basic tech sequence is fusion -> grav -> jump drive
(Assuming jump drive is somehow connected to manipulating gravity.)
Seeing as humans (and earlier pre sapiens hominids) have supposedly been around for 500,000 years (or whatever the actual number is) and only had agriculture and everything that spawned from that for 10,000 years or so and before sentient stone age humans there were supposedly chimps for however long we’ve only been at like TL1 for 1% of our total history or less so it seems to me the species who got to jump drive first would likely expand dramatically while most other species were either pre sapient or stone age sapient. The Traveller setting has that with the Ancients.
So you have your “angels” (or demons) from the skies, responsible for uplifting some species e.g. maybe even chimps to human, maybe keeping them as pets, maybe enslaving, maybe eating who knows. Then they collapse for some reason – maybe a virus, maybe they develop AI and it turns on them. Their AI fleets take over space and destroy all the Ancient’s planets that have any industrial capacity leaving surviving Ancients trapped on backwater planets where they revert in various ways – immortal space vampires, brain in a box, whatever. That provides a say TL15 to TL18 layer scattered and buried across thousands of planets. Then one of the uplifted species develop (Vilani in this example) and reach the fusion / grav / J1 spot and expand out.
This way you can have the players in a say TL10 to TL12 setting on the frontier of an expanding civ – say the Vilani drive tech only does J1 to maintain the frontier feel with deep space stations to cross gaps and lots of unexplored planets with weird ancient’s tech, occasional working ancient’s killer battle droids, maybe a working TL15 ship somewhere with a defective AI (i.e. it only occasionally tries to kill the crew), mostly low tech aliens, individual Vargr / Aslan planets but not space-faring yet, ancients ruins etc.