Can't believe I've just found this thread. Dad was a Sunderland pilot, so I am very interested. Even did some sketches of upgraded Sunderlands years ago. I was going to square off the wingtips, and maybe use the Allison T-56 as an engine replacement. Two of them, one on each inboard mount. On one outboard mount would be a honking great radar system, on the other a frighteningly bright searchlight. Also thought of squaring off the top of the rudder and ends of tailplanes.
Another mod would possibly be re-aligning the wings to remove the 4° sweep, including the thrust lines, necessitated when the installation of the Fraser-Nash 4-gun tail turret shifted the C of G rearward. This also led to a quirk in handling; sudden power reduction would cause a pitch up rather than down.
Others? Possibly a sturdier mounting of the floats, getting rid of the bracing wires. Rocket mounts under the wings, torpedo mounts, sonobuoy chute, radar guided turrets front, dorsal, rear, and possibly under the rear fuselage.
Take any idea you wish and have at it. And make my late father proud!