After World War II, various veterans of the vanquished (to put it that way) offered to design and make various military platforms. Only a few actually entered production. Kurt Tank designed the HF-24 Marut and sputtered-out Pulqui jet fighters. An Italian submachine gun, the TZ-45, found its way into the Burmese Army. Most fell victim to the same culprits: A combination of Cold War military aid and cheap Allied surplus sweeping them aside, while not being high-performance enough to beat them that way.
Yet for alternate historians/wargamers/writers wanting to add various wunderwaffe to their scenarios, it’s not that implausible that a few could slip through here and there. They’d mostly be out of the way unless history moved to that region (for instance, the Willy Messerschmidt-designed HA-300 would undoubtedly see service in the Arab-Israeli Wars if it entered mass production.) But they could still be done.
What would be most interesting would be the really exotic and large Luft 46 aircraft and wunderpanzer tanks. Those would probably need veto-able foreign engines (or other less obvious components) and would be no picnic to design. But they would be the most fun to play.