Biplane Jet Fighter
Ok, this is going to be ridiculed I know but, I have been trying to think of a good reason why a modern jet fighter would not benefit from some kind of biplane configuration.
Modern emphasis seems to be on manoeuvrability as opposed to outright speed yet surely with modern aerodynamics and wing sections, an extra wing would not cause too many problems with drag.
You would have extra fuel load, enhanced payload capability, greater lift etc.
If not a fighter then perhaps a good design for CAS as the increased fuel and ordinance would increase loiter time and firepower.
< Retreats and awaits comments along the lines of 'Stupid Boy'.