- The bulk of the Imperium isn't particularly rich material for exciting adventures. Yes, you can be involved in Imperial politics and royalty/megacorp shenanigans but the bulk of the inner Imperium is fairly homogenous.
Why would you think this?
Just because the inner sectors have never had much detail given to them by the publishers is no reason to think these regions are undynamic or all homogenous. The Imperium rules the space in between the stars, not the systems themselves. There's plenty of room for any number of strange cultures and unexpected diversity in Humaniti. You can put new minor alien races in wherever you like.
There are unexplored areas in every system, even in the core sectors and even after thousands of years of human occupation.
I am reminded of the introduction to the
Encyclopedia of Dagudashaag (put together by the people who worked on
Signal GK magazine):
"The picture many newcomers have of the sector is of a grey, featureless expanse of factory systems filled with grey, humourless Vilani. Nothing could be further from the truth. Dagudashaag is a sector of contrasts, from the soaring, fusion-carved mountains and glittering city-warrens of Medurma, to the floating dream worlds of Ushra, to the sulphur volcanoes of Lenashuuk, the macro-engineering of GaeWay's beantalk, and Kaurga's unspoilt plains.
"The social structure of Dagudashaag is equally varied. Arrive at any starport, and you will encounter the sibilant speech of the S'mrii, the strangely striped visages of Ziadd, the penetrating gazes of Scanians, and the towering, richly-robed figures of Aslan merchants."
- Most of the species are essentially humans in masks or furry outfits. Of the major powers that border the Imperium, 2 are human and the Vargr and Aslan are basically people with certain personality traits turned way up. The only power that feels genuinely alien to me are the Hive. There's nothing wrong with enjoying a universe full of vulcans and klingons but it's not the sort of sci-fi experience I want for my game.
The Zhodani seem the most alien to me, actually. What is it like to live in a culture where your thoughts are not private, everyone is happy with their lot, and there is no crime?
Having the most human-like races be near the Spinward Marches was a deliberate choice by the designers. And even the Spinward Marches has weirdness like the Llellewyloly, Sheol, Tashaki, and the Ebokin.
- The fact that the Imperium is surrounded on all sides yet all the other powers essentially have an endless frontier to expand into. By all sense, all the empires around it should have massively outgrown it and become vastly more powerful. Look at the craziness of the Zhodani coreward expedition. All those colonizable worlds along the corridor path of the expedition would dwarf the Imperium and there's no competition for those worlds.
That speaks to the Imperial system as being the most successful. The other races find it difficult to handle the space they currently have. The Imperium is "hands off" enough to claim a larger area, and diverse enough culturally to have a higher technology level than any of its neighbors.
I'm happy with the Imperium as it is. If I want to have campaigns with no Imperium in it there's plenty of places in Charted Space to put it. You don't even have to go very far - Foreven is right next to the Spinward Marches. There's no need to break up the Imperium.
In my opinion the Rebellion was a mistake, and Mongoose should go the way GURPS Traveller did - ignore the Rebellion and keep the Imperium intact.
Don't put in a new major race - there's no room for them. Just move over a few sectors from the Imperium and detail a whole new area of space if you want new previously unknown species to deal with.