This might have already been said somewhere within the 25 pages of comments, but if there is one thing I would like to see, it would be a solo and small group (2 to 3) book, which would bring more random generation and guidance and less reliance on referee imagination.
Obviously there will always be some imagination needed to create a scene, even solo, but for instance the Mercenary 2nd edition seems like a step backwards as far as content creation compared to the 1st edition (Ticket creation system and large-scale battle rules are completely gone. We chose to stay with the 1st edition specifically for those systems. I tweaked the ticket system to create random patron missions and payouts for my travellers. It needs a pro touch, but it suffices so far.)
As a nexample of other systems which could be included, what about expanded tables for ship encounter generation for descriptions of the ship. For example, a table to roll ship type (military, merchant, pirate, etc.) then size (100 tons, 200 tons, etc.) weapon loadout (twin triple pulse laser turrets, single missile rack, etc.) and then a quick formula for hull. Like a merchant might be 200 tons + 10 tons per turret (20 tons) divided by 3 (240/3), for 60 hull. That's entirely off the top of my head and probably a wonky formula, but you get the idea, and would give variation to the ships instead of just pulling another carbon copy from the book and sticking a weapon on it, or relying on having to fully build every ship encountered.
Stuff from the Campaign Guide could be updated and brought in as well. Some of the encounters and hooks are great, and when combined with a GM emulator, work okay, but it could use an update and a section for using it in solo play could be added. A Traveller-specific GM emulator and/or location generators would be a bonus. I've converted existing ones from other game systems for my own purpose, but a professionally done system from the people that know their stuff would be fantastic!
I found Traveller because the underlying systems lend themselves well for me to be able to play alone or with my two sons, and I'm certain I'm not the only one. While TTRPGs are intended to be played with others, it isn't always easy for some of us to find or fit in with a group, so we end up missing out on amazing adventures!