Why did you move the goalposts? Pumped-storage like Ludington are the most economical long term storage solution, they've existed for decades and unlike hydro generation, are not limited by suitable locations.
It's highly dependent on the kind of landscape you have though. Great for places like Switzerland, much harder to find a suitable place for it in Texas or Australia.
(Of course you can build pumped hydroelectric storage anywhere, it's just cheaper in some places.)
Hills or mountains make it significantly more feasible. Check the locations of most hydroelectric plants.
(By the same token, Sahara might be an ideal place to locate large solar power installations to feed the entire Eurasia, were it not for the transmission.)
And look at the map, they seem quite widespread.