Last I saw, at least 3 variants of USN P-3 aircraft were flying operational missions, and nowhere near the sea. I am sure that the P-8 will look to take on those roles in the near future, already there are pictures 'out there' showing the P-8 with various appendages underneath the aircraft.
P-8s have built-in provisions for installing the AAC (Advanced Airborne Sensor). This would give the P-8 an advanced E-8/JSTARS capability. These provisions are one reason why the weapon bay was moved aft of the wing, to provide an unobstructed area forward of the wing for the sensor and so that installation of the sensor does not eliminate use of the weapon bay. And USN is planning on phasing out the various versions of the EP-3 in favor of the P-8. In short, there's a heavy emphasis on multi-mission built into the airframe.
Perhaps we should also consider the E-7 Wedgetail. Between the P-8 and E-7 the basic 737 airframe covers a very wide mission spectrum. And with multiple customers from multiple nations for both aircraft, it would seem that both are going to be upgraded/updated for a long time to come, spreading the upgrade development costs over more airframes and across more national budgets.