Certainly the A320 at one point or another will become a freighter. The key being a drop in values for used A320s which added to a conversion cost would make the idea feasible.
Currently even older A320s have relatively high values which preclude any firm from considering investing in a conversion program. Even the oldest A320-100s are still flying with BA
and Air France, with no plans for retirement.
The next wave of smaller narrow bodies conversions are primarily centered on the Boeing 737 (300/400 series), with several conversion firms on the market. A potential likely aircraft following the B737 could be the MD
-80. The market values of MD
-80s have dropped which could be appealing for conversions. This would be especially true if a significant group of airframes are released onto the market from a large operator like American Airlines which would further depress rates and provide for a large pool of aircraft.