Realistic Batteries. It’s holding back a LOT of things. A lot of technologies are solved, but just require power.

Semi-Realistic Room Temperature Super-Conductor.

If that can be solved, the power density and efficiencies would just be astronomical… It would absolutely destroy multi-billion industries overnight.

Way-Out-There-Stuff If they ever prove out an actual functional EmDrive-like thruster, that would absolutely open up space travel to our species.