Good idea for a thread. Although, the design may be well under way but it will still be interesting to hear the ideas.
In terms of improving on the legend to produce a gun that competes with other high-ends out there, I'd list my improvements as:
3. Ability to handle brittle paint
Right now with the legend, the biggest thing it lacks compared to other guns is the weight and ergonomics of the gun. It doesn't fit in your hand as well as guns such as the Clones or Geos do. A substantially lighter marker would also be a big improvement.
Also, I understand that poppet valve guns will have some amount of kick so I'm not hoping for a spoolie. With the legend, it seems that there's certain changes that could be made to reduce the kick. Like I said, in order to compete with other high-end poppet markers, I think this is an area that could be improved upon.
Lastly, I'd say the ability to shoot brittle paint is key. On cold or rainy days, I've experience some problems when trying to shoot paint that other teammates with newer markers didn't have much problem shooting. Obviously the goal is to shoot paint that has as brittle a shell as possible.
The only other thing I would add is to, as Muhcaulk alluded to, spiff up the looks of the marker if you will. A solid color with black parts gets old after a while (unless you got the fade...but that still faded to black).