Biplanes & triplanes were used early on to compensate for high takeoff weight and underpowered engines. This is definitely an option, and always will be.

A flying wing is currently being used by HELIOS, which is powered by hydrogen fuel cells, and solar power. This is an extremely underpowered aircraft with high takeoff weight. A hydrogen turbofan would provide far greater thrust and allow hydrogen fuel for a much larger aircraft.

This has been done. Cryoplane has already flown using nothing but liquid hydrogen as fuel. Sodium Borohydride fuel may only have 66% as much energy per liter, but when you get rid of the big heavy cryogenic tanks hanging under the wings, you save bigtime on aerodynamic drag.