In realistic terms, such a trade would have to involve ships:
a) that a naval power like Japan could afford to give to Germany
b) that would be of significant practical use to a land power like Germany
Requirement (a) rules out Japan giving up any of its fleet carriers (and probably any of its smaller carriers too), as well as any of its modern (or modernized) battleships and battlecruisers. Japan could have afforded to give Germany some destroyers, but these would not have met requirement (b) – they would have been somewhat useful to Germany, but they wouldn’t have been game-changers by any stretch of the imagination. Cruisers might have been the best option for negotiations along these lines – say, one heavy cruiser, or a couple of light ones. Japan could have spared such ships more than flat-tops or battlewagons, and they would have been more useful to Germany than destroyers…most probably in a surface-raiding capacity in the Atlantic, similarly to the role that Prinz Eugen played for a while.
One qualifier, however, is whether what is being traded are finished weapons (constructed warships; constructed tanks) or simply weapon technologies. If I were Japan, I’d never trade a warship for a mere technology. Instead, I’d instead trade a warship for a very large number of finished weapons (remember that major warships take years to build and represent a huge capital investment), or I’d trade a technology for a technology. I think that Japan’s biggest selling point would have been its oxygen-fueled Long Lance torpedo, which in its time was the best heavyweight torpedo in the world. Arming German U-boats in the Atlantic with Japanese Long Lance torpedoes is probably the most useful thing Japan could have done for Germany’s war effort at sea, and it would not have required Japan giving up a single IJN ship. I’m not sure what German technology I’d have asked for in return – I’ll have to think about that one.
