In my games usually it turns out that Japan is small potatoes compared to Germany. Germany wins the war, Japan just makes things difficult for the Allies. This has been proven by our play when Japan has been totally abandoned by the Allies and all efforts have focused on Germany.
That said, in my experience there are some things Japan can do to make a late game Axis victory (even if Germany is KO’ed!) a possibility. The first Japanese turn, I believe they should buy one IC and a bomber. The American fleet at Pearl harbor should be crushed. If the British fleet in India is still around, it should be crushed, and if nothing has happened to reinforce India, it should be taken as soon as possible. The Russian space above Manchuria should be annihilated, as should China. All of this should happen as soon as possible. The IC should go in French Indochina and begin one of two strategies immediately - Either shoring up against an unconquered India or, if India is conquered, begin producing tanks to push into the Caucasus, Africa, and Siberia. While this is happening, the Japanese fleets (what’s left of it) should regroup off French Indochina or Japan and begin preparing for an American presence in the Pacific. The two bombers you’ll have can switch off between supporting your ground assaults and providing an extra bit of offensive power to your navy (thank god for 6 moves, they can be everywhere!). Try to maneuver the Allied navy into an attack on your navy, where your defensive fighers and carriers will pack much more firepower than their attacking equivalents. At all times, seek to deny them of the opportunity to destroy your navy completely. Harry them further by periodically invading Alaska or Canada. All the while, keep the pressure on Russia, denying them their rear area IPCs and even strategically bombing them in order to make Germany’s job easier.
In this fashion, I have played games where, when Germany fell, all of the Allies were still alive, and yet Japan came out of nowhere and conquered Russia, then crushed an Allied tank force in Eastern Europe. It was a monster, with over 70 IPCs and two monstrous navies. Granted, it still seemed like the Allies would eventually win, but it was truly an epic Japanese game all the same.