If you really want a challenge then try for as large a Vne/Vs ratio as possible.
I have the feeling that 3 or 4 is about all that anyone can design, from small to large aircraft.
Also to make the aircraft more economic, i.e. small engine yet still high speed, I think the L/D ratio of the wing needs to be as max as possible.. 50:1 would be nice, with a cruise at something like 180 Kts.... Could it be done with 100 bhp?
As for the instrumentation, ditch the round gauges (VOR ADF etc.) and just use GPS Navaids.
As for undercarriage.. A GPWS coupled to a quick deploying Wheel Set would be nice, and further help in the quest for minimum drag...
Am I just thinking about a posh motor-glider... ?