Here is a rough map of my hypothetical variant on the Traveller Imperium, inspired by a post by Daryen at the Mongoose boards. The idea is to "shrink" the Official Traveller Universe into 3x3 sectors, centered on the Spinward Marches, with most major powers present.
Here is the map:
Each major power is about a sector or so in size, enough for a serious campaign but still comprehensible in scale. As Tech Level is TL13 in most places, this means that you can get from Deneb (the Imperial capital) to the Foreven Frontier in 14-15 or so jumps - far enough for a frontier feel yet close enough for interesting political communications.
Now, on to the big question: what is the Imperium? I imagine it as a glorified cartel of the major Merchant Houses (AKA Zaibetsus), a bit similar to the Guild in Dune. The Imperium exists to facilitate commerce, especially by the Merchant Houses, and maintains a starport on most worlds, but does not intervene in local politics, or even small inter-world wars as long as the trade keeps flowing in and out. When a local regime threatens commerce, a trade embargo is usually more intimidating than the power-armoured Imperial Legions, though the latter are always ready to step on any local ruler who challenges the Imperial power. The Imperium allows for smaller traders, of course, but the major lines are owned by the Great Houses. "Nobility" is standing in the Great Houses; a Baron is usually a local Factor at a starport or his or her family; the Emperor is chosen by the Moot composed of the representatives of the Great Houses.
Oh, and no "Grandfather" in this ATU. There were multiple species of Ancients, including (but not limited to) the Droyne, Hivers and Darrians. And maybe also something more Cthulhu-esque. Who knows. But all Humans spread around space, and, eventually, colonized the space beyond the Great Rift, in the little-known days before the Long Night - rather than being transplanted by the Ancients. 299 years ago, the Imperium rose from the ashes of the Long Night and so did the local alien powers (who were also probably affected by whatever has caused the Long Night in the first place.
Most powers - Imperium, Zhodani, K'Kree, Vargr, Aslan - are TL13. The Imperium and Zhodani are mid-late TL13, while the K'Kree, Vargr and Aslan are lower-end Tech-13. The Hivers are TL15 (and sell late-TL13 goods to their trade partners, including the Imperium); the Droyne range from TL0 to TL16; and the Darrians are TL14.