This paper just ran an algorithm over a heightmap, and pinpointed potential reservoirs. This is a far cry from actually identifying feasible sites. The massive density of hydroelectric storage in the Himalayas, for instance, is far too remote to be developed at anything approaching reasonable cost. Not to mention, it still leaves swathes of places like Central north America, and much of Northern Europe without access to hydroelectric storage.

The European electricity market is already very interconnected, so I doubt that would be a problem there at least.