Would YC ever go the Techstars route and make it a city by city 'franchise'?
Seems unlikely. They sold their Boston office location and if you were maintaining a geographically distinct second location, Boston would be where to do it.
http://www.paulgraham.com/startuphubs.html
And if, as nearly everyone who knows agrees, startups are better off in Silicon Valley than Boston, then they're better off in Silicon Valley than everywhere else too.