MTU is purely homebrew and has nothing to do with the OTU, but in some ways it's a cousin to the Milieu 0 from T4. (Well, I did steal many of the aliens, but I ditched the references to the Ancients and any OTU history.) It's not an exact analogue (and it predates M0 by about 15 years) but the major power is a large pocket empire formed from a couple of smaller polities. It's now the 400-kilo gorilla of the region. This presents lots of points of potential conflict, as there are all sorts of neighbors - other, smaller PEs with varying attitudes towards the big ones, independent worlds with the same spectrum of attitudes, and a Big Bad enemy just off-map, rumors of which have been reaching the campaign area for some time. (I use the K'kree for this.
)
The main campaign area occupies a sector. I've mapped out many of the subsectors around the edges, but it's mostly unexplored territory as far as most people know. While I've never run an exploration campaign off the map as such, PCs did have to chase some pirates down in the relatively uncharted territory once.
The coolest thing about a setting like this is you can do almost anything with it - military campaigns, trade, espionage, pirates, politics, whatever. The scale is also at a good point where players can either be cogs in the machine or major players on the interstellar stage (or at least infamous!)
The biggest PE, the Unity, is a bit more "hands on" than the traditional 3I but has less to work with. I've also eschewed the whole feudal aspect for the Unity - I do use Social Standing, but it doesn't involve titles, fiefs and the lot. (There are a few feudal socities around, though.)
Hope this helps!
John