It'd help if you would say if you want to sleep in it or not.
For me - I want light weight, 37" tires, slide out cooler, and a slide out camp kitchen. Maybe a water tank? Propane and gas are easy enough to put on the tongue or rear. Sleeping in or on makes it more expensive / complicated.
Well, for me personally I'd probably put a large family sized RTT up top.
I like the idea of the clamshell design, but then the interior of the trailer can hold no, or very little cargo...
But in imaginary dreamland, which this thread is talking about

maybe there would be a way to build a trailer with a sleeping area on top of the storage compartment? Maybe a teardrop with 20" of extra of height, and have storage in drawers accessible at the bottom? I haven't really looked at AT's version of the teardrop yet, maybe they have another solution in place?
I really like the idea of a slide-out kitchen, if I was able to acquire the appropriate hardware, I'd build that into my "dream" trailer.
I started this thread to see if anyone had some unique ideas in mind that I hadn't seen yet. Granted, I've gone through a bunch of trailer build threads in here already, and there are plenty of great ideas. But, I'm sure someone, somewhere out there has got this cool little nugget of an idea that they may or may not be willing to share. Maybe this thread will inspire some intellectual generosity.
As for a water holding tank, I keep thinking about mounting a large one underneath the trailer's frame. Something like a big rectangular cube with baffling that wouldn't be too deep (for clearance issues), but because of it's large surface area, would hold a lot of water. A skid plate of some sort would likely be needed to prevent a puncture/rupture.
I think under the frame would be great placement to keep the COG down low when you're talking about water weighing approx 8.3 pounds/gallon and you have a tank that holds 30-40 gallons. Having 200-300 pounds of weight just above the axle would help keep the trailer planted pretty firmly on the ground.
The other little idea I was thinking about regarding keeping the battery bank charged, is a wind turbine on an extendable pole. Collapse it down when it's time to move, and set it up at camp if you're staying there for a couple of days. It would help supplement a small solar panel.
I haven't read of anyone doing this yet, but if it's cloudy or rainy for a few days, the S.P.'s aren't going to be able to charge very much...