Just incase this thread is still being monitored, here's an idea I've had rolling around my head for a while.
Something I think would be interesting to see, would be to take the concept and sound signature of the HD700, but to build it to flagship standards.
With the idea being that you have more than one flavour at the top of the line. So rather than having it be, as it was originally, a product that was aimed to be hitting a price bracket inbetween the 600 and 800 series (and also trying to appeal to both portable and home users) make a new version to sit at the same level as the 800 series. There are plenty who like the HD800 family, but there are also plenty who desired endless revisions, which were ultimately trying to make them into something that they were never originally intended to be.
The HD700 has a unique signature among sennheiser headphones, but perhaps it was held back from critical acclaim by being designed as more of a budget offering, and put in the difficult position of trying to please too many people at the time.
Of course, I am aware of the criticisms against the HD700, as indeed there were similar criticisms against the HD800/S. However, I have always noticed a general tendancy for people to have been more forgiving of the HD800 line in that regard... they might not have been perfect for everyone, but they seemed to offer enough overall to smooth over some of those misgivings, or at least more often.
Maybe making something new, with the HD700 kind of sound signature, but scaled up to the same level as the HD800's (or whatever new flagship equivalent will eventually replace the 800 line) would be filling a gap for those who still want the big 3D soundstage and high level of detail and separation, but happen to prefer a slightly less neutral and analytical approach in their listening, and prefer a slightly more fun and engaging and bassy presentation. Putting them at the same level would better allow them to compliment each other.
I might be in the minority here, but I think an HD700 S (?) would be a wonderful addition to my sennheiser collection.