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.