I posted this before I believe, but in my mind, it'll looks like this. The Mock-up is just a headset with that 3D effect in it. There would be a slider to adjust the 3D. Knowing Nintendo, they might even add a Gyro sensor in it.
A headset is more convenient than a 3D screen on the console itself, since you won't be forced to be in handheld mode only. With the headset, you can game with this 3D effect in both handheld and docked mode.
I believe there was once a time where people thought Splatoon came to 3DS, (which obviously couldn't) and the idea of playing Splatoon in 3D made some people hyped. (this is what I read on a Dutch forum back in the day). And yes, if this could become a thing, a Splatoon with 3D effect would be possible and quite fun. Aswell as other big titles from major companies. Like, we got Super Street Fighter IV 3D Edition on 3DS launch, imagine Street Fighter 6 to be in the same way. Or maybe if we got our hands on Red Dead Redemption II with the 3D effect. I'd also like to see the Call of Duty series having such feature. I won't say the 3D effect is a game changer, but visually it was marvelous.