If you are going to be racing when it is hot or cold out, insulation in the walls and roof, and even floor would be nice.

Some manufacturers offer a spring hinge on the ramp door to eliminate the cables. There's been lots of injuries from cables, so it's worth asking about.

A plate that is attached to the ramp door that covers the gap when you open the door. It's nice when you need to roll the car in or out on a floor jack, and for loading toolboxes or anything on small wheels.

Diagonal lights up in the ceiling/wall corner are nice and out of the way.

If you are going to haul a variety of stuff, e-track in the floor is nice, so you have plenty of tie down opportunities.

An escape door on the drivers side is really handy. Make sure you measure the height of the bottom of the door on the lowest car you will haul, and put that info on the build plan as the max height of the escape door bottom.

A "spare tire well" in the floor is a great place to keep seldom used stuff, or your spare if the cargo doesn't cover it. The last thing you want to do along side the road when you have a flat is unload the car to get the spare out. I got a spare tire well for stuff, plus a wall bracket for the spare.

I've had steel and aluminum trailers. I like the aluminum better. It's lighter and it doesn't get rusty. You pay for it up front, bu it helps the resale, too.

THe aluminum floor is nice, it's easy to clean. Make sure the ribs run fore and aft, not side to side. I actually saw one with a serrated aluminum floor running side to side. I asked the salesman how you would sweep it, he was flabbergasted by the question.

Here's a couple pics of my newest trailer: