I think that a cross-section only slightly wider than A320 and much narrower than B767 is not efficient for 6 abreast. The 2 narrow aisles would not be much help in disembarking, and would take up a lot of space. A single wider aisle would be better. But still problematic on short hops - waste of space. What you could do, however, is to have a plane slightly wider than A320 directed at long and thin routes.