There will be no artificial hills for pumped hydro. (Usually those are called "water towers" when used for municipal water storage under pressure.)
But deep subterranean cave and sub-ocean tanks for pumped hydro will be a thing. These make pumped hydro storage practical in radically more places than usually imagined. Combined with hill reservoirs, they multiply the storage capacity per unit mass of water.
A hilltop reservoir is, incidentally, an excellent place to site a solar array, which is cooled and more efficient by the water under it, and in turn radically reduces evaporative loss and biofouling in the reservoir.
But deep subterranean cave and sub-ocean tanks for pumped hydro will be a thing. These make pumped hydro storage practical in radically more places than usually imagined. Combined with hill reservoirs, they multiply the storage capacity per unit mass of water.
A hilltop reservoir is, incidentally, an excellent place to site a solar array, which is cooled and more efficient by the water under it, and in turn radically reduces evaporative loss and biofouling in the reservoir.