Thoughts.......
#1: No one has yet executed the soft tail design properly yet. Siren comes closest in my opinion.
#2: 29"er Scalpels have been test ridden in the field for at least two years. The Cannondale/Dorel company have not seen the numbers or dealer pressure to go forward with it.
#3: The proposed cross between a Big Mama and a Dos Niner is intriguing. If Salsa pulled it off in a package that provided 3 inches of travel in a laterally stiff, efficient chassis, then the JET 9 might have a serious competitor for the 29"er XC/endurance racers dollars.
#4: The RX was a bike that mikesee reviewed along with some other rigs on this forum several years ago. As I recall, he pronounced the RX as the stiffest laterally, and the stiffest in terms of suspension feel. A racers 29"er FS design. Maybe it should be resurrected? Maybe the Superfly 100 is a bling version of that bike?
#5: The Pivot 429 seemed like a bike that had XC race written all over it when I rode it. Stiffer feeling suspension than anything I had ridden in its class with a rigid lateral feel. Maybe it is too heavy/expensive for some tastes, but I thought it would fit the bill very well for what is being asked for here. I don't think having 4 inches of travel is a bad thing for a bike in this category.
#6: In my opinion, the bike in question needs to be light race bike, (sub 6lbs for frame and damper), be laterally rigid, allow for out of saddle performance on par with a hardtail, and take the edge off the rough stuff while keeping the rear wheel in contact with the ground. Basically, be everything good that a hardtail is and have none of the drawbacks of riding a hardtail. This is if we are talking about a racing bike for XC and fast paced shorter endurance events. We already have great long distance endurance rigs, light trail bikes, and efficient hardtails.
I think the problem that any bike that fits the parameters I am talking about, (JET 9), has is that it gets taken out of context by the riding public, is found to be defficient, breaks, and disappoints some buyers. Light weight can be an addictive elixir, and when you get away from the racing courses and start trail riding, or AM riding on these sorts of rigs, the sparks start to fly. At least that is how I am reading what I see here and in other forums.
In the end, it may be a case of "be careful what you ask for"!