Here on the Kernel page, we frequently read about features that are maybe two or three cycles away from getting merged. What about setting up trees for future versions that would naturally turn into mainline after a new kernel is released? It could cut down on merge window chaos considerably.
(For example, we're at 3.6-rc6 right now; we would have a 3.7-rc0, 3.8-rc0, and maybe 3.9-rc0, and when 3.6 is released, 3.7-rc0 becomes 3.7-rc1.)