My ten....

Newer cobra trailers have trays attached to the wing dollies that bring the
cleaning materials in them down the back when rigged. Nice touch.

I wouldn't be without my hydraulic jack. Much much easier.

I store my probes inside the clamshell at the back, safe from damage.

Easily swappable number plate. Install mountings at the standard cobra
pitch so crew can easily attach their number plate.

Very strong hinges, fittings, and locks.

Solar panels or wind turbine that can drive a fan or dehumidifier as well
as charge batteries. I wish I could power a small heater to keep the
temperature above freezing.

Screw jacks are a pain. If you do not want to prop with the door (the rear
of my trailer sits on tyres) then fit drop down stands and wind the front
up on the dolly jack to drive them into the ground.

Single axle better than dual but fit Tyrons to ensure tyres cannot come off
when flat.

Double skin or insulation to ward off condensation.

Maintenance free exterior - hate washing and polishing.