First of all the Axis player I will face likes to keep the USA and USSR out of the war as long as possible to build up his forces and destroy the forces he is already at war with (China, UK). I am already a big fan of Russia involving himself with the Chinese front by keeping the Siberian forces north and send a mechanized team to help out Yunnan. I think this might be an interesting gambit / strategy against J1 and J4.
However I just got an idea, as the Russian airplanes can reach Yunnan in 1 turn. Yunnan is next to India the most important zone on the Pacific board, if China can keep it early by making it Fortress Yunnan, Japan is in trouble as China become a big headache. Yes Japan can ignore China and go for the Siberian route or through India, but lets just assume that it is much easier for Japan to just deal with China while also dealing with Russia and the UK.
There is nothing you can do about Japan taking out Yunnan in J1, as the Russian fighter and tactical will not change the odds much against 3 Infantry, 1 Artilery, 1 Fighter, 1 Tactical and 2 Bombers.
However you could surprise the Japanese player by going all in on Fortress Yunnan before J2. This will make taking Yunnan too costly for Japan and will either cripple Japan very early in the game or make China spiral out of control early in the game.
R1: build 3 fighters, move 1 tank + mech towards Yunnan, move the Leningrad fighter to Moscow
C1: retake Yunnan and reinforce with all your units + 4 infantry you build (now total of 10-12 infantry on Yunnan)
UK1: make unprovoked action, declaring war on Japan by reinforcing Yunnan with 2 infantry, 2 fighters, 1 tactical (now total of 12-14 infantry, 2 fighters, 1 tactical on Yunnan)
R2: move all aircraft + tank and mech to Yunnan (now total of 12-14 infantry, 1 mech. infantry, 1 tank, 2 tactical bombers, 7 fighters on Yunnan)
Japan has too few land troops available in J2, it might not have all its aircraft within range so soon because of the Russian 18 infantry stack is still there and can not handle Fortress Yunnan without losing most of its aircraft.
The Red Tigers (Russian Aircraft) can reinforce India when needed, or meet up with the large Russian stack north and wipe Japan out of the mainland. It is also fast enough to be back in the motherland Russia for the final battle, while in the meantime it gave Japan a lot of headaches!
For Russia this first turn investment in the Red Tigers means ofcourse 10 less infantry in R1. However lets look at it this way: if you can ensure Japan will not be able to reach Russia, you stopped the second front. Through all three possible routes, Siberia, China and India, those 5 fighters + 1 tactical will be of great value in addition to the infantry stacks. Russia is to Japan what German is to Russia, so why not abuse this? Those 10 infantry less will not make a difference against a determined Germany and at least you prevent yourself from being surrounded from all sides.
India has the support from the Red Tigers, there will need to be some calculation, but this might even be enough to stop a possible India crush from Japan?
Japan that loses the mainland has no chance of winning in the pacific, so is forced build up land forces or try go through Siberia or India, allowing the USA to focus on Germany.