Fusion power. In fact, we (probably) have done all of the basic science that's needed to build fusion power plants, the only thing that remains is a very expensive development process.
The process of fusion is pretty well understood: you cram a bunch of very hot hydrogen together, it fuses into helium, and releases a bunch more energy. You use that energy to boil steam, and after that a fusion power plant looks just like a regular coal, gas, or nuclear power plant.
The problems are technical, not conceptual:
1. How do you reliably start a "cold" fusion reactor by cramming enough hydrogen together and getting it hot?
2. Once the fusion reaction is started, it generates so much heat that any fusion reaction naturally wants to blow itself apart. So how do we keep that fusion reaction stable enough that we can keep feeding it new hydrogen and keep the reaction going?
These are really questions of the geometry of the reactor, of the mechanisms that need to be built, of how many and how strong of magnets do we need to confine the reaction, etc. 435
We will have to do a lot of very expensive modeling and simulation, then build prototypes, then go back to the design stage and improve those prototypes, then build more prototypes. Eventually we get to the point where all of the apparent kinks are ironed out and we will build the world's first pilot fusion plant. That will be another big learning experience, and probably 5-10 years after that you will see the first generation of Mark 1 fusion reactors starting to be installed around the world.
The only problem is that all of this is terribly expensive. Unless there is a clear and convincing reason to spend billions to trillions of dollars developing this technology, it's much cheaper not to. From a technical standpoint nuclear power is just as clean as fusion power would be, and in the modern era there's much more of a focus on making renewables incrementally cheaper and more efficient rather than building a whole new technology. (Note that the problem of nuclear waste and nuclear contamination are more political problems than technical problems.)
This is what caused one of the early fusion pioneers, <PERSON>, to say, "Fusion will be ready when society needs it."
The science is done, but the very expensive development has never been funded. Fusion has been "20-30 years away" since the 1950's because we got to the point where we can't go further without massive investment, and that investment just hasn't happened. 435