My top ideas to expand Traveller:
1. 2300AD - A Sol sector book. Its great being along the various arms. I would love to see what Earth in 2300AD is like, what Mars is like, the Moon, the Jovian moons, etc. There are job opportunities chances for whole campaigns in good ol'Sol Sector for Travellers. It would be fun to know what modern Europe, or Asia looks like, adventure and intrigue along the USA/Texan border, going up a beanstalk to the Earth low orbit to hop a ship to the Moon. Getting involved with helping the colonists on Io deal with a questionable corporation. Piracy hunters around Saturn. Whatever, just be fun to be in Sol. Maybe even have some ideas on how to create say a French Traveller who would get French specific benefits when they mustered out of their careers or what the various Space Forces looks like in 2300AD to provide potential adventure seeds and character background depth.
2. 2300AD - An updated Tools of the Frontier to reflect the current change to the rules with the Core Book 2022. I know we can use the current Central Supply Catalog as well as the equipment construction guides in core book. Still it would be fun to have an updated 2022 version and even one that might blend some of the items from the Robot Handbook into the 2300AD world.
3. Traveller - I know there is High Guard 2022 coming, but it would be awesome to get a book or even a folio filled with maps of ships for regular table play and the PDF is optimized for VTT play. It would be awesome to consider doing what some other publishers are doing and release some maps on a semi-regular basis that allows for one to use wet/dry erase markers on it and minis.
4. Traveller - Scout Service/Colony Development handbook. It would be fun to have a source book on how the Imperial Intersteller Scout Service does their job and how that is combined with developing a Colony. Discovery of a new planet and scouting it out to help decide what is Amber or Red zone for colony. You could include some introductionary rules for creating Xenofauna and Xenobiology, as well as adding to the Xenofauna and Xenobiology to try out before doing a full on Xenobiology book like others up thread have asked to be created. The other half of the book could deal with how a colony gets developed and its stages as allowed within the Imperium Rules, Aslan society, Vargr society, or even Hiver concepts of how colonies are developed in stages and typical equipment they might use. Maybe even expand to explore what it would take to make a Class X,E, or D starport in a colony world.
5. Traveller - Trader/Smuggler handbook - For those that enjoy the trading aspects of the game. Expand on how trading works. Tighten up the rules that are found to be confusing from the core book. Offer what some of the major trading companies look like in the Imperium and how trading works at some of the typical starport. Some of the ideas with how to role player broker better than "roll a D6 against your broker skills...and you find a trading house at the port..." give the GMs some flavor on what a starport trading facility looks like. Is there better trading houses that handle goods and are cutthroat about the costs to the small little place at a starport with one person always looking for someone to take on a cargo for any price to any place? Is it a giant floor where you walk into computer bank and offer up services and get bids from the various agents at this starport? Again flip side of this, is the more illicit material; give an idea of what smuggling looks like in the Traveller Universe. What and how does Law Enforcement and the Imperial Navy deal with smugglers? How would the players go about finding a smuggling job is it a "Show me whats behind the counter..." or is it "You arrive at the Bar of Broken Mesons and need to seek out Jonesy the Broker. The clients of the bar don't look like law abiding...."
6. Traveller - The Frontier Conspiracies. Don't make this a box set, but maybe a set of loosely inter-linking modules. Where the players are drafted by a mysterious patron to investigate and prevent a potential frontier war. Do one for the Zhodani, Vilani, Solomanis, etc. Where the players find themselves basically employed as spies or maybe even some pulp like hero such as Simon Templar type or Mystery Inc/Scooby Doo type; A module where there are rumors of war and the players spend time to prevent it from blowing up into a major event. Where it doesn't matter which one you pick up there is a chance that you report back to the patron and it ends with a rumor about another threat to the frontier. Each one could have a different threat such as to prevent an assassination of a major noble, find evidence that the liner attacked in space wasn't done by the other side, chase down pirates operating near the frontier who are forcing a stronger Imperial Navy presence is violation of a treaty, escort important diplomats to a neutral system for talks to resolve ongoing issues. Whatever, it doesn't really matter what the story line is, but it can provide a fun set of adventures where intrigue and a cafe named Rick's gives the players something to look forward to for session time.
Those are the ideas that I have at the moment to help expand the Traveller universe.