Barring another invasion storyline or finally ending the brand split once and for all, a major title unification is one of the few "megaton" plots left, and probably the simplest to pull off. It's also something that can be fairly easily reversed if the need arises.
I'm thinking of it more as a short-term angle here. The stars are aligned in such a way that it could really work as a quick surprise pop if they need it, but with Rock/Punk on the horizon and Lesnar still having a few appearances left, it might not be sustainable for too long before getting overshadowed by angles already in the queue. But I can't think of a better way to nail down Punk as a Cena-level modern legend, which is what they need to be doing/what they should've had done by now.