Britain's unrealized 24-inch torpedo potential

Which underused weapon technology had the most development potential?

Britain's 24-inch torpedo, because it might have become a powerful Allied heavyweight weapon comparable in effect to the Long Lance.

The Royal Navy already possessed a 24-inch torpedo associated with Nelson and Rodney. That weapon was a missed foundation for a much more capable heavyweight torpedo. Continued development might have produced an Allied counterpart to the Japanese Long Lance in destructive effect, though not necessarily through identical technology. Such a weapon would have added formidable striking power to Allied surface forces.