I love speculation! so here's my theory for Wii U/3DS successssor ;)
What I hope to see is a 2 in 1 gaming device (yes like many people think): a handheld that interacts with a ''TV box'', basically a Wii U but with the gamepad being the size of a DS and which fits your pocket
- Games will use cartridge or any alternative that can hold lots of data while still being small. They will be inserted in the handheld.
- The ''box'' which will let you play your games on the TV simply acts as a signal converter between the handheld and TV. With no input lag using Wii U's technology.
-The handheld will be able to produce Wii U-like graphics on its small HD screen (720p), and 1080p when being streamed through the box, to the TV. The handheld will also act as a controller, and playing on the TV will add some features (like in Windwaker HD for instance), but games will not focus on dual screen gameplay.
-All Nintendo developers will now only have to focus on 1 platform, thus, we might not have any game drought. And no need to buy 2 gaming devices.
-Price point could be 300$ or 250$, we're talking about Wii U graphic capabilities but scaled down to fit the handheld (Vita size or smaller).
-You can use your Wii U gamepad for local multiplayer games, or the pro controller. Wii remote compatibility might be removed.
-Most likely no backward compatibility with games.
-No more 3D screen, Nintendo knows most people don't care about it (I think).
-Back to the GBA design, meaning only 1 screen, but with more buttons (Wii U gamepad with smaller screen, and two 3DS circle pad)
-No more Wii name or DS name. Something like Nintendo Revolution or simply ''Nintendo''.
-4G for online multiplayer on the go.
-Uses Nintendo Network account, which will let you download all the VC games of your Wii U/3DS.
-Release it with 3 big 1st party title day 1 for maximum market penetration.
*What if it fails? How are they going to make money?
-How could it possibly fail when you have all Nintendo games in one package instead of 2? It would also be a lot easier to market. Remember Nintendo hardware sells only on the back of their games, nothing else.
*But history tells us Nintendo will release their next handheld in less than 2-3 years!
-Maybe they will take a different route this time and release it in 4 years? Or perhaps it's going to be possible to release it in 3 years...
If the theory doesn't make sense, please tell me so. I guess they could also make some kind of VR machine, but I really doubt it, I see it more as an accessory down the road.
PS: Nintendo hinted us when they said they were going to merge their console and handheld departments, why would they do that in the first place? They are also expanding their Kyoto HQ! see photos here for proof:
http://nintendoeverything.com/photos-of-nintendos-new-kyoto-headquarters-under-construction/