Here's what I thought of when i saw your request: a business person who wants their trailer to look good, and not look bizarre, in the looks-are-important suit and tie environment of an office.
My imaginary business person wants the trailer to be waterproof to protect his laptop, documents, etc. Roomy enough to carry lunch, laptop, some papers, etc. He knows it might get dirty on the way to work, so he wants it to hold on to water/dirt as little as possible.
If you wanted it to be really "stealth" in the office, you could make it look like an airline carry-on suitcase which has the little 10cm wheels as well as a pair of standard child bike wheels (standard 16" ?) which can be removed and hidden away. Maybe instead of a center mounted retractable handle, it has a side-mounted retractable handle that can be used as trailer tongue or carrying handle.