I'm certainly not a trailer expert. But for me, the main reason I'd ever own a trailer would be to give me the convenience of quick camp setup (like the RTT discussion recently) without having to be tied directly to my vehicle, which also serves as a recreational tool while out and about. So for me, the minimum requirements would be:
1) Would need to hold a fair amount of potable water (at least 13 gallons), propane (at least 5 gallons), and possibly a deep cycle battery
2) Would need to be able to go wherever I want to take it
3) Would need a decent amount of storage to hold/store things I couldn't/wouldn't otherwise bring if I only had my X
4) Would need to be able to serve as a fully functioning campsite - Some things would be more necessary, and some would be more of a luxury. Example: 12v water pump would be nice, but gravity fed system would be fine. On board water heating system would be nice, but cold water would be fine. But at a bare minimum, I'd need to be able to use it to cook, clean and hold everything I'd bring.
As far as dimensions and what not... I'd say the smallest package that can successfully accomplish all of the objectives is best. Width for me would have to be the same as the truck, or less. I certainly wouldn't want a wider trailer, but I don't think I'd want it too much more narrow than the rig either for stability. Once I had all of the "essentials" worked out into a size, I'd weigh the non-essentials (luxuries) against how much bigger/more expensive they would make the trailer, and decide which ones I'd want and which ones I wouldn't based on that analysis. But that's just how I am... I always consider the overall value of any item or feature against all of the other specifics... I don't necessarily "need" anything, it's just a matter of how much I'd want vs. how much I'd be comfortable spending on them.
I think maximization of space and having things be able to serve as many functions as possible would be key (like a multi-tool or a swiss army knife). It could take some extreme engineering to utilize every square inch in a productive way, but it would be totally worth it in the end. Basically, you should make this trailer, but without the $45K US price tag
Needs to be able to serve as a fully functioning camp-site, so that my vehicle coul