This idea is brilliant. The problem of finding outside countries to run the cities is perhaps less difficult that it seems. The Hanseatic League and Lombard Lea

This idea is brilliant. The problem of finding outside countries to run the cities is perhaps less difficult that it seems. The Hanseatic League and Lombard League cities prospered in large part because they were free from feudal control and could develop local, non-agrarian industries. The cities need enough outside control to allow the economy to develop but not necessarily complete sovereignty like Hong Kong. As for making it economically worthwhile for the "overseer" nation and particularly individuals, perhaps that could be solved by sharing in the high economic growth rates that successful charter cities would achieve. If you get that right there would definitely be takers give the current low growth rates in most industrialized nations. (Paul Romer is one of my favorite economists of all time so maybe I'm biased.)