A fleet of Q400 multi roll water bomber is certainly worth a look at.
In addition to dropping retardant it can ferry in fire crews and supply's, evacuate persons if required, operate on unpaved runways.
Operating costs would be far less than for the B737, they can if need be sold off to a secondhand market as a pax aircraft or a freighter as the water bomber tank is external attachment and not a structural modification.
They carry 2,640 gallons of retardant (the B737 is 4,000 gallons), so if you scatter a few crews with planes in the hot zone airports they could mobilise faster to the fire while the others are on transit to the fire.