As a Ref, I would find the following suppliments useful:
A book of ships and small craft that the playerz might encounter, or acquire. Complete with deckplans and art.
This is definitely a useful one - compare and contrast CT S7: Traders and Gunboats with S9: Fighting ships. A book of adventurer-sized ships, not necessarily any more than a dozen or two designs, provides a useful background piece with resources that actually might get used. Once they have deck plans and stats - and they are in the size and power range that works for bunch-of-yobs type adventures - then you've got them on a low-prep basis for most things you might use them for.
New sophonts, that Travellers could encounter or play as.
Less useful, I think. Although a handful of alien races with some mechanical distinction is useful, they're less useful if they're not readily playable, or are too similar to some other race in their mechanical effects. IMO for things like races you want something that sits in about 5-10 different niches, and after that you're starting to get into diminishing returns. The exception to this is if you want to do something like Star Wars, where the large proliferation of alien races is a core part of the look-and-feel of the universe.
More patrons, a la 76 Patrons.
I think you can never have too many patrons or adventure hooks, although the first edition of 760 patrons has shown that they still need to meet a reasonable quality standard to be much use - a lot of it felt really phoned-in. 76 good, well thought out ones is more useful than 760 bad ones.
A book of several worlds that a Referee can plop down into a campaign. Complete with adventure hooks and interesting planetside sights.
This sort of thing is quite useful, although a significant amount of effort to do. However, I think a small sandbox of a couple of subsectors with 20 or 30 well thought out and written up worlds would be quite useful as a low-prep setting. Particularly if you had write-ups for factions, various sites drilled into in detail, and encounters, rumours, patrons etc. for all of the starports or other places your players might hang out. Not a five minute job to do, but I think it would be a useful game aid.
[ . . . ]
Short adventures that a ref can run in case he hasn%u2019t prepped or if the players decinde to go off into left field.
And, in this day and age, you can make maybe a dozen or two such adventures, but skin them for various different worlds via electronic publishing as PDF. In conjunction with the detailed sandbox above, you have a low-prep campaign pack for a busy referee with a job and 2.4 kids. Add a few modules for specific campaign arcs - aimed at 6 months play each or so, then your sandbox becomes quite a useful low-prep-for-busy-refs product.
Now, this is quite a substantial body of work - it could easily run to half a dozen volumes - but it's quite high value. The scale doesn't have to be the whole Imperium, just enough space to run a sandbox in.