I'd vote for going after the Soviets. Stab them in the back when the Germans attack, and secure Siberia for the greater glory of the Emporer...
the SU is, i think, a better target than the US because they're closer, would be distracted with a major (even life-threatening) enemy closer to the SU centers of power, and would prove an easier target...
the problem is building up your ground forces to counter the SU; it'll take a while to research the proper doctrines, raise more divisions, equip them with proper brigade attachments, etc. I think a trap some Japan-players fall into is following the course of RL history and challenging the US. The US will, without a doubt, outproduce and outmuscle any realistic Japan (meaning, no cheating to create uber-Japan), and to paraphrase Yamamoto, you'd have to march clear to Washington, D.C. to end the threat the US poses.
Split up the SU with Germany, and then hope a dominant Germany can invade the UK, and then create a powerful enemy for the US to face across the Atlantic and split their forces...
just my two cents, though....Excellent updates, as usual. Interesting that you got all of Indochina in the event; did you press for more than the historical claims, (meaning, Madacasgar?), or did you play it low-key to avoid any problems?
Lemons