Isn't there already a successful low-cost operator in Brazil called Gol ?
With regard to Ryanair-style remote airports in the US - there are possibly a few around the New York City area, eg. Newburgh, White Plains, Islip (and maybe other Long Island airfields), the last two of which are already used by Southwest. Also, Hanscom for Boston is a possibility, though there are big noise issues there, maybe Long Beach for Los Angeles and Oakland for San Francisco (both used already by JetBlue), Williamsburg/Newport News for Norfolk or for Richmond (used by AirTran), and there's also some white elephant airport east of St Louis which is used by virtually no-one (I forget its name). Maybe even Northeast Philadalphia and Trenton/Mercer as stand-ins for the busy southeast Pennsylvania/central New Jersey area. Baltimore is already a big Southwest hub (close-ish to Washington).