Having owned the LCD-2, HD 800 and T1 for a while now I see all three have their own advantages and disadvantages. I think the HD 800s have a physical design that feels very well thought out compared to the other two, which it should since it was in development for so long. The cable is light, the cups are big and spacious and overall the comfort is the best of the three for me. Note that my head is big, so I use the biggest size on all three headphones and the HD 800s win big on comfort. That the headband and earpads are easily removable also made it easy for me to clean them.
On the other hand the T1 feel slightly more luxurious with their materials and I like what the inside of the cups look like. The design does feel a bit lazy though since it's basically a premium version of the 770,880 and 990 design. The earpads do very easily touch my ears and they are only just big enough to fit comfortably.
Then there's the LCD-2 which is pretty heavy and do have a very functional feel to their design, but not nearly as refined as the HD 800s. I am speaking only of the construction and not the audio here. The cable is pretty heavy and unlike the HD 800s the cable noticably pulls the headphones down on your head. I think the earpads are more comfortable than the T1s and even with the weight the comfort is good, but not really to the level of the HD 800s. The sound quality I think had both pros and cons compared to the HD 800s as they do play deep bas extremely well, but it lacks the very open and spacious feel of the HD 800s which I guess are thanks in part to their very open design and driver alignment.
So just from a headphone construction standpoint I would like a headphone that somehow combined the extremely open and futuristic design of the HD 800s with planar magnetic drivers and it would have easily replacable and cleanable earpads and headband while also having a high quality cable that didn't add a whole bunch of weight to the headphone needlessly.I left out other high end headphones here since I have only had limited exposure to them.
What would you like to see in future high end headphones? While the sound quality is of course of the highest importance, the physical construction and just the small details that went into it start to matter a lot after a while if you're going to live with them for years.