A battery tech that is as high density as fossil fuel with little to no self discharge that can be used for short term and long term storage. Ex: it can be left at half charge for years without being a problem. This would be good for grid level battery storage where you could charge it with renewable energy that is not predictable and it would act as a long term "capacitor" to smooth out the ups and downs of solar/wind generation, as well as grid demand.
Something like this would not only be good for grid, but for vehicles too. Perhaps even airplanes. The biggest drawback to renewable sources is the unpredictability, so the answer is a storage tech like this to deal with the ups and downs. You would have these batteries all over the grid along with renewable energy plants and together they would provide enough power at any time whether it's sunny/windy or not. If the batteries are running too low then you could fire up the nuke plants. You'd do this when there is less than a week worth of charge left in the batteries, for example.