Not strictly combat as it never hit the battlefield but the Rockwell XVF-12A was incredibly interesting.
It was very ambitious, attempting to combine the AIM-7 Sparrow armament of the of the McDonnel Douglass F4 Phantom, a 20mm M61 vulcan with 639 rounds, loadout capacity of 2 AIM7 sparrow and either 2 AIM-9L Sidewinder AAMs *OR* 4 AIM-7's, in a very small package that was both Mach 2 Supersonic and featured a unique wing design designed for very rapid breaking, and a reheat system designed for tremendous acceleration, coupling an extra pair of rear intakes to reduce the fuel consumption for rapid acceleration.
Unfortunately the engine couldn't produce the thrust for vertical flight reliably in all weather conditions due to a failure in the design of its thrust nozzle and the heavy weight of its construction necessitated by the moving parts in pair and would be superceded by the less ambitious Hawker Siddeley Harrier GR1 we all know today. The team later collaborated with British Aerospace to do something quite interesting:
In 1988, British Aerospace also had two very interesting concepts: The P1214 and the P1216 both of which were split boom configuration fighters designed by the same team who designed the Harrier in order to neutralize the exhaust backwash problem causing vibration in the rear fusalage (sometimes called the "three nozzle problem" where more than three stabilizing nozzles pointed downward will cause severe kinetic disruption of a hull).
Of the two, the P1216 was tested very extensively right until a scale technical demonstrator. The team were later absorbed into Lockheed Martin as a major function of the aerodynamic concerns of the American F22 -- pushed by British Prime Minister Margaret Thatcher who believed lending expertise would reduce end costs and strengthen US/UK relations - basically meaning she wanted to buy some 22s and A10C's and outside of Israel and a few places the US are notoriously stingy with their planes despite the UK selling Harriers to them to sweeten the deal.
The software control suite developed for the 1216 (particularly its advanced logistical systems, radar and six axis radar based image aspectation and helmet linked visual systems) were all further developed for the F-35 which inherited an enormous number of patents and... Technical hiccups.