Sigh... So hard to choose...
I guess I'll take Russia, since Japan's already taken and Mexico... well the enemy of my enemy is my friend (unless someone starts a war in Europe and as a result Mexico and I end up sharing a border...)
Btw, I know my history pretty well (AP WH+wiki+curiosity), so ya... (summary: Japan developed because they noticed that they were so far behind the "West" and, unlike in China the "lords," liked change. They ended their era of isolationism and basically copied Western machinery. They soon fired the samurai because they did not want to adapt to a world where you didn't need to know how to use a sword and where almost anyone could be a relatively skilled soldier. However, they did not have many raw materials, and as a result had to import heavily. This is also one of the reasons why they invaded Indonesia (which they won't have to do in this timeline = less dependence on Western powers at mid-early stages of industrialization, = more power, = if Russia starts out as slow as it did IRL, Japan>Russia, = also Japan > China if it's as neo-confucianist as IRL, = Japan=Asia, = Mexico fights Japan, = if Japan gets allies, it becomes #1; else, Mexico becomes #1.)
Right?
EDIT: Just noticed, Japan would have to fight Prussia over dominance of Indonesia = one less possible ally and a lot more possible enemies.