...the highest probability goes to an invasion of North Korea by...China. Winning favor and international accolades by cleaning up its own neighborhood. Especially when its inhabitants have outlived their usefulness. Passing on the opportunity to be the "liberators" of North Korea before the Western World does it for them is a very tempting prospect.
It also holds the benefits of unanimous international support, real-time testing for conventional Chinese military logistics, and (given the inevitable outcome) sending a strong signal to the west (in addition to its trading partners) regarding its willingness and ability to control its interests.
Everything else in the region appears to be in (more or less) equilibrium.