But I still think there's a huge capacity gap between E75 and 73G. Something in the 105-110 seat range would plug it nicely.
They could likely put in orders for E190/195, the 190 holds 94, and the 195 106 in 2 class standard config. They would probably have to operate those as mainline.
I don't really know what sort of used aircraft would become available in the seating range they seem to need more. The 318/736 come to mind but they are very high CASM aircraft.