It makes absolutely no sense
It makes absolutely no sense to increase the electric-only range of the Prius Prime to 240 miles. At that point, just make the battery a bit bigger and dump the gas engine completely.
No, it's far more likely that the battery would be shrunk to 1/5 the current size. Give it a 50 mile electric range, lower weight, lower cost, and more cargo area. That's more than enough range for the average person's daily drive.
And that's the key to electric vehicles, hybrid or not. They're designed to be charged every day overnight, and most people won't use more than 20-40 miles. Then, for those uncommon times that more range is needed, the question is how to provide that. An electric car shoves a much larger battery in the car -- lots of expensive weight that's mostly unused. A plug-in hybrid shoves in a gas engine -- proven technology that has to be maintained. Putting both range extension options into the same vehicle is pointless and only confers the disadvantages of both.