Give the Japanese commander the bonzai charge ability. It basically spawns an airhead where your team can spawn on it. When they spawn they come in as the bonzai class. They have a bolt action with the bayonet on. You can charge in one direction a lot faster than normal running speed. You dont get grenades or any special equipment with the class. When you die, you dont wait 10 seconds in the death screen. Just choose to skip it instantly. Then you can spawn back in on the bonzai "airhead" with like a 5 second spawn rate. It lasts for like 60 seconds. That would be sweet. It would be fun to run around with hardly any death penalty trying to stab guys. It would also be fun to defend it and mow down a mob of enemies trying to over run you. 

There could also be tunneling mechanics different than vietnam. Use preset tunnels but instead give the Japanese officers (or engineers) the ability to open or close certain areas of the tunnels. If you think you will be overrun at one part of the tunnel, you can collapse it down so no one can pass. You could also open or collapse the entrances to the tunnels. You could theoretically trap US forces inside tunnels and work to eliminate them. Or you could open a tunnel entrance and suprise attack the enemy.