I don't know about a kneeling coaster. You don't want to put that much pressure on the knees, and restraining a person would be hard in that position. I think that the motorbikes will take off once a major US park gets one. Same with Eurofighters.
As for the next innovation, I have no clue. What I'd like to see would be the "rocket sled". Take Kingda Ka, but lay everyone down on their backs. That would be fun. Somewhat impratical, but fun.