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.
Batteries are the big one. Can you imagine how many people (homeowners/renters) will go out and buy a tiny 100W panel knowing that even though it will fill a battery with energy very slowly, they can still bank on it for a week?
Right now we have batteries that can survive about a day, using a modern solar panel system with inverter (~1000€). Imagine when we have batteries that can store weeks of power.