I'd take a DR350, bump it out to 400CC. give it the DR650's oil cooler, NikaSil plating, 5gal tank (don't have to have it full for single tracking, to keep the weight down) with the modern chassis/suspension of the DRZ400 while retaining the old DR's 6 speed box.
It must retain screw and lock-nut valves for easy on the road servicing (no parts required for valve adjustments) and F.I. to cope with todays shitty gas and changing altitudes.
...that's my perfect 200+ mile a day bike for single track woods exploring and highway droning @ 75 mph.
Oh yeah, I'll also take the old 91 DR650's seat, luggage rack, skid plates and mini-windshield WITH the kick starter.
I.M.H.O. Suzuki at one time, has had all the parts and capability (for cheap) but it never wound up all on one motorcycle.
So, what are the attributes that make the perfect bike for me? (above)
Simplicity, reliability, serviceability and flexibility.
As for touring two up...I.M.H.O. that just nixed 1/2 the fun of the adventure and escape. (you guys can have your big bikes for those needs)