I was looking at some of my sci-fi RPG stuff recently, and was curious about the use of cargo space. There's kind of a running joke in Battletech, in that almost every warship has a large dedicated cargo bay even though it doesn't necessarily need it for its mission role. And while you could make up logical uses for it, like spare fuel or ammunition, the ship's requirements already factor those in. It's kind of like how many of Jackill's ships include an arboretum regardless of the ship's role. Ships in Starfinder with expansion bays also have cargo holds as a default option, with the ability to add mission specific bays (some of which take up multiple slots, like hangar bays for small craft). With regards to Trek, I can see some designs as being better suited to carry bulk cargo if necessary, like the Miranda family and maybe the Constellation design (which has a lot of bays in the saucer). FASA would occasionally have older designs being sold to civilian interests, after having any proprietary Starfleet equipment removed, so it's interesting to think of that as one option. I'm also of the mind that dedicated cargo vessels, like the transport/tug family, would still see some use to haul specialized cargos to colonies and other areas.