I'll just point to the following two questions and their associated answers for background:
A comment then led me to the BBC news article Claim made for hydrogen 'wonder material', quoted extensively in the second question. One line in the discussion about the possibility of hydrogen remaining in a metallic state after removing the initial, incredibly high pressure (about 5 million atmospheres or 500 GPa):
The US space agency is also fascinated by the material. Already super-cold liquid hydrogen makes for a very powerful rocket propellant, but the dense metallic form of hydrogen promises to deliver really colossal levels of thrust that would enable huge payloads to be lifted off Earth.
So I have to ask: Has (theoretically) metastable metallic hydrogen been considered for use as a propellant?
I am guessing that if hydrogen could remain metallic at ambient pressure, the density would be significantly higher than liquid hydrogen, overcoming one of the disadvantages of LH2 - giant, heavy tanks. But is there thinking that the thrust would also be significantly enhanced?