I'd try somewhere in Europe as everywhere is very close and very populated.
England has plenty, as do Germany and France.
Birmingham in England has a pax of 9.5Million for 150,000
Nurnberg in Germany has a pax of 4.5Million for 100,000 (about the most pax you can get for 100,000)
Klefavik in Iceland has a pax of 1.5Million for 100,000 (there's no competition, so long as you can get enough range on your planes-London Stansted is 1172NM, JFK is 2590)
Hope this helps.
Personally I would take Nurnberg if you strictly had to stick to 100,000
