I suggest:

over length .. tip-to-tip.. so that it fits inside you garage (with the car on it)... mine was 20"... fit inside my 21" deep garage.
open center... so you can work under the car... very important and convenient when you need to do so!)
extra long and wide ramps.. preferably that pull straight out/in
a beaver tail where the rear tires will just sit flat in front of the angle
extra lighting (led)
quick release removable fenders (or low height so that you can ope your doors)
low flush storage unit (to keep the ties, etc)
flipable hand crank winch (or mounted electric)
load leveler attachments and bars
4-wheel electric brakes
extra max-width... for big wide tires and easier to drive up
flush mounted tie hooks
front jack top rest the trailer

steel trailer is traditional and less costly, but aluminum is lighter and more expensive... pending how much you'll be trailering (but aluminum typically is full bottom... not open center... and with not as many choices in size as steel are offered)

also... get a spare tire

the above was the one i had built by JB Trailers (Elkhart, IN).... they recently went out-of-business.. was a great trailer for 15 years of seasonal usage,.. towed throughout the midwest ... kept it in my garage with the car on it (worked under the car whenever needed)... I sold it to a friend.. then it quickly got stolen! I hope to get another similar to it (closed would be nice but.. storage becomes the problem).