Honestly? A major breakthrough in fusion, or to a lesser extent, any other clean energy. We’ve decarbonised a decent chunk of the world’s energy profile, but there’s a strong financial incentive that politicians are vulnerable to protecting oil and gas, slowing down further decarbonisation. Batteries and supercapacitors also could do the trick.
I personally think we’re on the slope of enlightenment - quantum computing no longer attracts as much hype as it used to, but in the background, there’s a lot of interesting developments that genuinely might be very important.