Fuel, water, and a deep cycle battery under the floor/deck to keep weight low.
Virtually all of these trailers are way to fragile. I fully expect to roll such a trailer many times in its life so I will say an external structure that will sirvive such activity's. All internal storage designed to carry the whatever so they will sirvive 360° impacts.
Tow/extraction attachment points on all corners, including the top.
Lights all directions, top and bottom.
Fuel, Water, Battery, Tow points, Lights. My top 5

I would also add jack mounting points, and in my design I have a winch.