New Propulsion Systems: Metallic Hydrogen

Kerbal Space Program 2, according to a video on their official YouTube channel, is going to be adding other fuel sources, along with rockets to accommodate those. Metallic hydrogen is one of those. And that gave me an idea: that's something Simple Rockets 2 should have. Yes, we don't have interstellar travel yet, but we have interplanetary travel--and other fuel types and engines for that would be awesome! Nuclear thermal engines are already in the game, as are ion engines engines. Why not go further with exotic/near-future fuels and propulsion systems? Metallic hydrogen would be the first step.