Between using the Captured Planet fractional orbits, placement with less bias for gas giants, and the extension of size in T5, the only exoplanet I recall outside the current system is Godzilla, which is supposedly a superterrene at around Size 17 or 18. The exotic speculations about atmospheres that come out of the exoplanet programs largely fit into Atmosphere B and C, if for different reasons than GDW envisioned.
A rule or two explaining why skimming from Hot Jupiters is a bad idea would be nice, if only because you aren't going to be very efficient due to Gas Laws, might overload the cryogenics of your fuel system with gas that hot, and, of course, might just cook off all those pesky organics in the ship, including the crew...
Traveller takes enough inspiration from SF that it is likely to keep the eggworld atmosphere code, though a lot of Astrophysicists think that can't happen. Mesklin, Medea, and Jinx would disagree, of course.
One thing I keep in mind is the example of Victoria in an early JTAS, and use chemical gradients to mitigate or liven up the Tainted atmospheres.