Fully & Quickly Reusable Rockets (refuel & refly, like jets)
Obviously both SpaceX and Blue Origin are the leaders here, but once they do it the other majors will either have to build the same thing or drop out of the industry.
There are so many things about space that we just assume are true, but are actually only true because access to orbit has always been so expensive. If we can get the cost to reach orbit down to a multiple of the fuel cost, then so many more things are possible.
We finally get large satellite constellations for low-latency Internet all over Earth. We get space stations and O'Neil cylinders. Moon bases and fuel depots on Titan.
At the same time, firms like Made In Space are working on in-space construction so you can build radio telescopes in space with arbitrarily large dishes (10 km, maybe?). Eventually we build mirrors that size too.
Basically just those two things are the only barriers between us and a solar-system-wide civilization like in The Expanse.