Am I the only one who flies the Behemoth? All I see out there in play are trains and container ships, once new players get the cash to buy them. This super transport idea might be okay because of the seriously crappy flying characteristics you intend, but I cringe at the thought of an even larger cargo hold out there. Single-system trade runs would need to be scrutinized.
A pipe dream of mine, which I'm sure is beyond the game engine... commodities that require specific kinds of holds/containers. Imaging a 20,000 unit Supertanker which can only haul commodities flagged as "liquid". Or consider modular ships that can mount passenger "cans" with life support for those 2500 people jammed in the hold; seems funny to pack passengers into a hold that just was emptied of H-fuel or Scrap Metal.:)
"To gain a hundred victories in a hundred battles is not the highest excellence;
to subjugate the enemy's army without doing battle is the highest of excellence."