Would be interesting historically indeed, but there's always the catch that for each new boat introduced we have to throw out another one. I wouldn't mind losing one of the Typ II variants or even the Typ IX B for a new boat, but as with the Typ XXIII it is doubtful that the effort is worth it all with the Typ I A. From my perspective the German-built boat for the Turkish Navy that was pressed into service as U-A would be interesting to play since it survived the war
http://www.uboat.net/boats/foreign_ua.htm