All obvious places may have hydro plants already, but not all hydro plants are reversible.

The country point is interesting, so the market is countries lacking topography suited to hydro plants? How big is that market really?

And considering that pumped hydro is currently the only viable grid scale energy storage mechanism (inb4 but li-on), those countries currently either 1) share a grid with neighboring countries that do have pumped hydro, or 2) forego energy storage altogether and use peaker plants like everyone else. If you do 1 you better trust your neighbors to never go to war with you, otherwise you better have 2.

So the market is: countries lacking topography suited to hydro plants, who currently share energy storage with neighbors who they don't trust, and who for some reason aren't just building peaker plants (which are essentially just normal fossil fuel power plants) and are holding out for a non-extant technology to be developed.

I'm not much of a capitalist myself, but even I can tell that that's a bad investment.