IJN Subs IS a possibility now
 

The work to get on the opposite side is done with the add on. Sure modeling needs to be done on the people and boats but in reality it is easier to actually command an I Boat. Modders?????
Oh and can we lose the US Diving alarm and go with the standard alarm for the U-Boats - I have no idea what a I Boats alarm sounded like - anyone else?

Wonder if it's possible to make the IXC playable in SH4- there were also several IXCs in the Far East. Might be a matter of transferring the relevant files from SH3- does anybody know?