The hostel scene is saturated in Australia, New Zealand, Fiji, Argentina, Chile, Singapore, Malaysia, and a lot of the more popular Central European cities so I wouldn't recommend starting a hostel there. But there are a lot of major destinations with no hostels.
In Europe there are no hostels in Toulouse, France; Waterford, Ireland; Leeds, Lincoln, Ludlow, Norwich, Sheffield and Shrewsbury in England. Milan - Italy's largest city - only has three hostels; and Turin only has one hostel.
As mentioned earlier there are several major US cities with no hostel. These include Baltimore, Columbus, Des Moines, Detroit, Kansas City, Memphis, Milwaulkee and Pittsburgh. There are plenty of other smaller cities in the US that could also easily support a hostel.