You want to go high altitude? You want less engines. Just look at the Lama and B3 Squirrel (altitude record for helis I think).
This should give best power to weight you can get (with sufficiently powerful engine obviously - efficient because you only have single gas generator to drive). Clearly you need to strip out any excess weight too (no redundancy like twin hyds for example).
I would also design the rotor for efficiency at high altitude (it'll fly like a pig at low level though). You would have to design control phasing for the altitude (Lock number effects) - you see this going wrong when flying a "sea level" helicopter at 20000ft - forwards back left and right all get skewed around. I would also increase control gearing/control power as everything gets a little sloppy on the controls.
I think a lower rotor speed would also be a good idea. The speed of sound is lower at altitude (ie lower temperature) so tip effects would kick in earlier. Longer blades might cause problems with tip effects so I think broader chord is the way to go.
Just some thoughts