If ED is an extrapolation of real world practices, and things in ED generally seem to be, then presumably the future would see us using stuff similar to the ULDs (unit load devices) which airlines currently use.
ULDs are standard-sized containers which conform to IATA (international air transport association) and ATA (air transport association of America) guidelines. Sometimes the designations are IATA or ATA ones, but these are simply different names for similar containers, for example, an ATA LD-8 container is the same as an IATA Type 6A. But regardless of which designation they use, they have to meet certain criteria, for example, they have to state the relevant ULD Type Code, the maximum gross weight (MGW) in kilograms and pounds and the actual tare weight (TARE) in kilograms and pounds. Beyond this, there are additional letter and number codes which further identify a particular container as specific to a task, for example, some containers are refrigerated, some for livestock, some certified for the main deck, some only to be carried in the lower cargo hold, etc, with much of this to do with fire suppression capabilities on the aircraft. Because of this, there are still some cargoes that cannot be carried by air of course, for example if you have ever ordered some types of paint online, you might find them arriving in the mail with a big prominent sticker on the packaging stating 'not to be carried by air'.
Naturally, one can take this kind of thing too far in a game, after all, we do not want to be simulating the job of a loadmaster or cargo ramp agent, nor do we want to be signing off a 'zig-zag' (the nickname for the load sheet given to the pilot to sign). So I think that a simpler system would be to - as Angus suggests in a post above - simply imagine that the 'space transport association' or some such governing body of the future, has decreed that 1 ton is equivalent to a ton of pure water on a 1G planet (or some other such convenient commodity) and everything else for transport is measured by that designation. This would be sensible, because one would assume that cargo bays on spacecraft would follow the present day practice which airliner designers follow, whereby they literally design the aircraft around the shape of containers such as the LD-8, which is angled on the bottom sides in order to conform to the shape of an airliner's lower deck hold. This is why aircraft from rival manufacturers tend to look very similar in dimensions, for example the Boeing 737 and the Airbus A320.
In ED, we might therefore have such add-ons you have to buy for your ship in order to transport certain goods. Although there are going to be specific passenger ships in ED, one could also include upgrades such as cargo bay life support, specialised fire suppression gear, airtight sectioning, additional escape hatches, G-rated crash deceleration protective netting and bulkheads, etc, all of which have a broadly similar real world equivalent on todays cargo aircraft.
In other words, some additional interesting realism in ED, but without turning a fun space trading game into a boring techno-fest. And this could even add an element of gameplay too, for example, you could take a chance on carrying potentially flammable cargo without the recommended fire suppression gear fitted to your craft and risk a spot check fine if you get inspected, or possibly your ship being more likely to explode if fired upon under such circumstances.
