First, I don't think we reached the point, there graphical improvements get to small. There is a lot of improvement possible, compare games to movies.

But while the power-difference between WiiU and PS4/X1 seems big, still all three use withered tech as defined in the OP. So I think there is a difference possible about the graphics capabilities and the price.

On the lateral thinking side of things - I think most concepts for getting into games are explored. Maybe Virtual Reality changes this, but at the moment I'm not convinced Virtual Reality is really a thing. But if virtual reality is coming, it is clear that it will radically change the way games control. Without it I see more or less a saturation point. This is visible in the controllers. Even if the WiiU gamepad seems different with the touchscreen, it basically is similar to classical controllers (similar buttons and sticks). I don't think the gamepad is really a big innovation. It seems at this point, that this is exactly trying to apply lateral thinking, but with no real aspects of changing the gaming. You probably wouldn't like a car today, that isn't steered by a steering wheel. There is a natural optimum for a set of tasks, and it seems to me, for controlling games this natural optimum is nearly reached. Except Virtual Reality is a thing.

I feel different, and really the gamepad is only a slight variation of this design, but everything feels different about this. The Wii-controller was a real change, but Nintendo didn't stick with it, and game-devs badly supported it, so it seems this change isn't happening.

Diminishing returns is a fact of life.  At this point all consoles are very powerful computers, and it takes a massive increase in power to show even modest gains in grapical output.  The more the technology matures, the smaller the difference will be between competitors without a signifcant price difference.

Diminishing returns not only applies to hardware, but to software development as well.  Each developer has to weigh how good they want the game to look to how expensive they want the game to be to produce.  Even if movie level CGI during in game playable segments becomes technically possible, will it be affordable to produce?

Lateral think does not just apply to controls.  It can apply to game design as well.  Hell, it can apply to marketing, business models, the technical aspects of game development...just about anything really.  There is no reason to think lateral thinking should be a thing of the past.  That will just lead to stagnation.