How about Prop fans -got trialed on DC9s/MD80 test beds but went no further.
Offset nose gear, OK they did fly commercially on Tridents but no-one else adopted them. Giant aerodynamic shock bodies a la CV 990 come into this category too.
Nuclear power -I think there was nuclear powered B36 but only one and the idea was never revived.
Composite -not in the carbon fibre sense-aircraft .ie one carrying another. Short Mayo flying boat/seaplane combo and B36 (again) carrying a couple of miniature jet fighters along with it
Many weird Brit innovations from the 1950s as already alluded to in earlier posts