As per the LinkedIn profile of former studio boss Michael Mumbauer, Sony's not so top secret San Diego studio is currently expanding "upon existing franchises". The project is said to be a AAA action-adventure game that utilises proprietary engine technology. "This team was assembled to expand upon existing franchises and craft all-new stories for the next generation of gamers." Why hello there, Uncharted.
The developer not yet made public is indeed widely rumoured to be working on some sort of PlayStation 5 title set within the universe of Nathan Drake, but not necessarily casting him as the lead protagonist. After the events of Uncharted 4: A Thief's End, there are a number of different routes the San Diego team could take. Most interestingly, as noted by IGN, Mumbauer's LinkedIn page contains mention of an Uncharted 5 by way of the "Featured" section. The user is able to tag videos to their profile, and the former boss in question has done so for a YouTube clip featuring franchise speculation. By no means any sort of confirmation, but an interesting tidbit nonetheless.
The studio's continued existence was called into question late last year as a couple of high profile names announced their departure, but that speculation was quickly stamped out by former senior game designer John Bautista. At the time, he said his exit was due to "personal circumstances" and "the studio is still there and the project is still ongoing", as far as he knows. Let's hope an official announcement of the game takes place this year.