Big problem with just plain "Imperium" is that it could be confused with Warhammer 40K, especially in my part of the world.
Grand Empire of Stars is quite good. But, for me, is a little vague and unevocative. Pity "Ziru Sirka" is so First Imperium.
*cough*iridium throne*cough*
An Iconic cover that fans will recognise is easy. Slap on a type s scout in front of a starburst and we will know its Traveller. We need a decent name that will draw in new people, but still be a recognisable traveller universe.
Chartered Space covers it, but sounds a bit lame if you don't know Traveller.
Traveller;Prime is quite good. My second favourite infact....
Something catchy including the word "Humaniti" would also be quite good. But I can't think of anything off the top of my head...
Iridium Throne is no less tied to one polity than Third Imperium is. Further, it's LESS informative.
I know, but I wasn't going for informative so much. Like I said a cover with a scout and starburst will let Traveller fans know its Traveller. There can be a little subtitle saying "the Original Traveller Universe" and a new player can read the back cover for more if they are interested.
It's just something that might sound a bit grandious and interesting to someone new, while at the same time sounding OK when discussing it with people who have played for a while.
Third Imperium is fine when you have played for a while. Infact it suits better than Iridium Throne. But if I had never heard either and was wandering a store looking for something new, Iridium Throne would sound a bit more exotic and interesting than Third Imperium.