When the Pacifica was in development FCA already planned the e-rear drive for awd so it is being tested and packaged to fit the platform. Unfortunately there isn't much room for more batteries so energy density will need to improve for range gains - or you could go the earlier Prius route and reduce performance to increase efficiency. I vote for improved battery tech.
At allpar.com, there was an examination of the Pacifica's PHEV system. The final passage discussed how they might incorporate AWD:
"Our sources claim that the AWD system will be set up by tapping the same basic system, but with a single additional motor for the rear wheels, allowing AWD vans to have Stow ’n’ Go.
(The normal PacHy uses dual electric motors driving the front wheels).
I have no idea how a single motor powering the rear wheels would be powered, but I'm going to guess by the same general principle as Toyota's Synergy system, which is via a much smaller 'helper' battery mounted somewhere other than the Stow n'Go bays.
It would also seem that such an AWD system could theoretically operate as a typical hybrid where the rear wheels would engage/disengage to lighten the burden of the front, ICE-powered wheels.
In effect, you'd get a hybrid where, instead of an electric motor occasionally helping drive the front wheels of the ICE, it would be out back, driving the rear wheels. Then, when the system sensed the front wheels losing traction, they'd automatically engage at that time, too.
You wouldn't be able to plug-in and operate as an EV. But you would get the increased fuel mileage of a hybrid, with the added bonus of AWD.