If the allies (presumably excluding the USSR) wanted, for some reason, to restore the German monarchy, Friedrich Wilhelm would have been absolutely the wrong candidate. He was so unacceptable that even in 1918, when people were urging Wilhelm II to abdicate, they expected to pass the crown over Friedrich Wilhelm to his son, under the regency of a different prince (Eitel, IIRC). In 1945, after FW had flirted with the Nazis (mostly, but not entirely, because he saw them as a vehicle through which to restore the crown), his chances would have been even lower.
Hirohito was necessary in Japan to provide and legitimise transition. That wasn't necessary in Germany, which had already experienced democratic republican government in the 1920s.