If you went with the C130J engines and nacelles, a new wing, added a new lower lobe to the fuselage and lengthened the fuselage, you could probably get away with the same basic fuselage diameter, which pays big benefits in development. The lobe could handle a larger weapons bay, move the sonobuoy chutes out of the rear cabin, permit more avionics to be placed below the floor and, with modern smaller avionics, allow the same or smaller crew to work with more comfort. It would allow the carriage of heavier weapons, additional fuel and permit longer patrols. Derated engines would permit a lot more flexibility in cruise while allowing a greater all up weight for take off.
All while keeping mostly within the existing vehicle size envelope and fuselage shape, simplifying development and keeping the very economical turboprops and low altitude capabilities that come with them. Always thought he P-8 to be too fast for a lot of the work the patrol aircraft have to do outside finding subs.