This doesn't exactly jive with the existing look of Traveller, but I can think of several practical factors that figure into the overall look of a ship.
1) Use of internal space -- cost and operating efficiency dictate that ship components are always in a intense competition for real estate. And then there's always cargo. The most efficient use of hull materials vs. internal space is spheroid. Wherever possible, spheres will be dominant, especially when gaseous or liquid cargo -- like fuel -- is involved. Cuboid shapes will predominate in other areas, as with solid cargo and sophont environments.
2) Protection -- space is a rigorous environment and vessels will require shapes that maximize the effectiveness of a protective hull. Strong rigid structures such as hexagons might be an important motif both in minute details (hull patterning) and in the larger structures of the hull as armour is angled to deflect explosive force and avoid absorbing it fully. Until meson weapons predominate, power plants, command and control facilities (the bridge) and the like will be placed in the most well protected areas -- like the exact centre of an armoured sphere.
3) Drives -- the TL of the maneuver drives of the ship are probably the most dominant design constraint. Reactionless drives, depending on how one conceives of them, completely do away with the need for angled thrust, and the overall motif completely changes. Directionality -- the notion of a specific front or rear to a ship -- becomes passe. It can now move in any direction with equal ease.
4) Heat dissipation -- the TL will dictate how heat is radiated by the ship. The higher the TL, the higher the efficiency of heat arrays and the less they need to be sticking out all over. Perhaps, at a certain point heat dissipation can be done across the full surface area of the hull without compromising the quality of the armour.
5) Stealth -- if you believe it is possible, it will impose a very distinctive design aesthetic, depending on how it works. I like the idea of stealth, but for me it is bound up with the heat dissipation issue.
6) Sensors, weapons, communications. The shape dictates the operating scope of any one sensor, weapon or communications array. A full 360 degree field of view on both axis will be impossible to achieve even in the best case scenario, but cost and operating efficiency concerns dictate that these systems get as close as possible. Evasive maneuvers against direct fire weapons often involve using the structure of the attacker to limit the number of weapon systems that can simultaneously find a lock at one time. Fighting ships will want to be able to easily target as many weapon systems as possible on a single point from any angle.
