Screw-down stabilizer jacks like the ones used on Cobra trailers now are great. When I bought my Avionic in '04 they only had one at the back. If there had been an option to get one on each corner I would have taken it. As it is I crank down the rear one and set the front of the trailer on a pair of jackstands when it's parked in its usual location at my airfield. Nice and stable even when the trailer is closed. I prefer that to putting strain on the tailgate.

As Bert Compton mentioned a place to put the canopy would be GREAT! I have an ASW-15 and the canopy is kind of awkward to deal with when rigging.

The over-center mechanism for raising and lowering the fuselage dolly ramp is good if you can assemble the glider easily without the ability to make fine changes in the height of the fuselage. In my case the only thing I would need to raise and lower the fuselage for would be to raise and lower the gear so the over center lever approach would work fine.

A third rail and ramp for the tailwheel would be nice, especially if the glider has a heavy tail. The 15 isn't heavy but being able to get the fuselage in and out without lifting the tail would have been worth the extra cost.

Contrary to what Bill White says my preference would be for a number plate holder which makes it difficult to remove the plate because where I live the trailer has its own VIN, title, insurance and number plate. I assume he lives somewhere where the trailer plate must be the same as the vehicle towing it.

Electrical connections for the lights should be whatever is most common with your fellow pilots. Either that, or make sure you keep an adapter or two on hand in case you need a retrieve!

For a similar reason, despite the advantages of electric brakes I would still go with a mechanical surge brake. It might not work as well, but at least it will work with any vehicle they're hooked up to.

A bit more ground clearance than is common with newer trailers would be good too as long as you don't get too carried away. The one thing my old trailer had over the Avionic that replaced it was the ease with which it could get in and out of driveways without bottoming out. Some sort of skid plates at the rear should be installed anyways, just in case.

The slide out under floor storage drawers seen on newer trailers are a fantastic idea.

Don't make it "just wide enough" for the glider to fit. Give yourself a few extra inches space. It makes getting things in and out MUCH easier. My old trailer was a non-clamshell type but because the builder resisted the temptation to make it small it was actually very useable. You could actually make your way from one end of the trailer to the other inside the trailer when the glider was in it. Too bad the wood it was made from had rotted so much.

I have yet to make up my mind when it comes to how the fuselage is held in the dolly. A single strap attaches easily but it only works to prevent the fuselage moving rearward in the dolly. Two metal arms swinging up from the dolly and attaching to points on the fuselage (e.g. with bushings in the ends of the arms slipping over the pins sticking out of the fuselage and held there with a spring clip) seems more secure but you would have to get the fuselage in exactly the right place on the dolly before they would line up.

Looking back at trailer history it still amazes me how long it took for some people to figure out the "fuselage in nose first, wings in root first, right wing on the right, left wing on the left and tailplane in a bracket above" arrangement.