I really doubt power metal will be it. It doesn't have the coolness that it needs. The masses like thier metal to have very certain characteristics. Look at popular heavy music. Punk. Thrash. nu-metal. What it all has in common is what the next big thing will have. Well, everyone has to be able to connect with it, for one. Lyrical themes preteens and teens associate with. It has to be simple, so people can understand it. That rule has exceptions like tool, but in general, it has to be comprehensible at a low level. It has to be somewhat catchy. I must say, that among all of the ideas, country metal sounds the most feasible, although it is a stretch.