What I'm most interested to see from a 3rd party is designs for Vehicles, especially ground vehicles, grav vehicles and aircraft.
The reason for this that if I were to use setting information that I didn't make myself, I would want it to be "canon" Traveller. Likewise, I for character clasees and aliens, I'd want them to fit in with all the other Traveller information I already have, so I'd be leary of buying that sort of thing from a third party.
However, I can never get enough statted out vehicles. I need them for adventures all the time, and if they are made with the "official rules" Mongoose or T5 or T20 or whatever, and they work with the system, then they are as good as "canon". I might be interested in ships too, but to a lesser extent than planet-side vehicles.