Graviman,
You mentioned "variable RRPM" in your post. A new thread entitled [Variable Speed Rotors and Prop(s)] has been post, to place this idea into the public domain. You may wish to critique it.
Slowrotor,
IMHO, there is only one choice ~ composites.
Composite construction is no longer the future, it is the present. Aerospace is into composite construction in a big, big way and the prices get lower and lower.
The composite that absorbs moisture is Kevlar and there is probably little reason to use it in a rotorblade. Man-made silk is a future composite under development. It is said to have the greatest strength to weight ratio of any known material.
Dave