In the days when A400M was 'FLA', one of its intended roles was as the intended RMPA. However, the design requirements for a sturdy, austere base transport aeroplane with a freight floor and those of an MPA with a need for sensors and weapons carriage were, to an extent, mutually incompatible.
The A321 would, however, be a reasonable platform upon which to base an MPA.
Although whether it would be affordable is a different matter.