How about we kind of change the definition in which we are using it.. It should be more in the lines of "What is a next gen experience"
In my opinion... Gameplay itself is traditional. There's always a shoot, a melee, a conjuring of some type of power/perk etc. However, gameplay itself true evolution is in the game experience..
For example, take Titanfall. Take away everything from Titanfall and it is your basic FPS. You shoot, you run, you jump, you melee, you conjure. However the themes of the game that are put together is really what makes the game what it is. Now you can say that, it isn't something other games haven't done. But that's just it, it isn't, however the full experience is what sets it apart.
Next gen gaming to me is that. It's always going to be a little bit of the same because the core mechanics will never leave, it's all about the experience that will matter. Scale, adds to that. Back then with limited hardware we could only travel so much, explore so much, do things outside your normal story with limits. Now it's giving more life to a world, an environment.. Giving life to a theme that we have never heard of before.
By theme, I'm specifically taking about things like Bioshock.. Mixing Underwater adventure with steam punk and a cloud Atlantis, giving a game a minimalist styling about a lone traveler with little to no words, exploring the adventure of a hacker who can control the entire city, basically throwing together a themes that you have never seen or heard of that makes it different.
However when it comes to Sequels, I think that's when things get blurry. Saying things like it's more of the same makes no sense because the game is built off of a theme that it's going with. It isn't like Nintendo where they have a universal character that can jump into different genres when they want. Sony couldn't take infamous and turn it into a side scroller or a FPS. Because the game wasn't built like that. However them adding more to the experience is how they achieve that. Whether that is adding more powers, branching off onto another storyline.. Making the map bigger and destructive, etc etc.
So in my opinion, that's what a next gen experience is.
TLDR Edition: Gameplay is traditional, however game experiences such as themes, settings, design, and fresh ideas is what will define next gen *imo*