Don't expect ND to start yet another new IP. There's that documentary about this game (removed from YT by now, I think you need to buy it?) and they talk a lot about how many things they had to figure out.
They originally did not want to have infected people in the game, they should have just died - but that way the player would not have had to face the direct effects during the game. So they needed the Infected, then they had to spend a lot of time figuring out how they should look like, how they should behave.
Then there was a lot of research on how nature would take over the cities, what would happen if canals aren't drained and how it would ruin buildings and flood streets. Or research on survival, how and what to make from what components and using which tools.
In short, developing this game involved a lot of extra work, in particular the world building and finding the art direction using lots of saturated colors and beautiful environments which goes against most post-apocalyptic settings I've seen, except maybe I am Legend.
So Naughty Dog can now do Jak&Daxter, Uncharted and Last of Us games for the PS4, why would they need more? It makes every kind of sense to capitalize on this IP pool, instead of investing another 1-2 extra years on developing something new and then the "standard" time of developing a nextgen game. And from the above, the world of Last of Us has the most left to explore, compared to yet another Drake adventure...
