It's exciting to order a trailer custom made, the chance to get it just the way you want it, as opposed to "off the lot". We had a 20' trailer made when we ordered our Kubota
L3830 years ago. Here are a few thoughts:
A black paint job will be much more forgiving down the road, but to each his own there.
I don't think there's such a thing as too many tie downs. You never know where you're going to need them.
Back up lights. If they don't come standard, I'd ask for them. They can double as loading lights.
I'd ask for enough room on the A frame - tongue area for a large toolbox. Great for holding tie-down chains, binders, wheel chocks, etc. etc.
As someone else mentioned, make sure the jack is mounted far enough back to lower the tow vehicle's tailgate, even when turned at an angle.
It's probably standard procedure, but it's important to have support legs under the ramps, and the longer the ramps the milder the angle when loading and unloading.
I think you said you're planning on making it wide, but I had the fenders spaced out wide enough to be able to drive a pickup truck on.
Good Luck!