Okay, so, this is an interesting idea, but I'm still not convinced it would sell. The 3DS successor system, I mean. I feel like it would face similar issues that the Wii U and Vita face. It sounds like the general idea behind it is that it's basically a portable console, and that Nintendo's games with more handheld-esque design philosophies would stay on the "mass-market handheld". The market doesn't want a portable console, which is a big problem that the Vita has. The 3DS has both games with console and handheld-esque design philosophies that do well but I'm not convinced that the system would do well with just the console-esque design philosophy games. And I also think that the 3DS successor system would probably end up being too expensive to sell well.
Also, side question, but how would local multiplayer work on the thing? Does everyone have to own their own system? Can you buy controllers that you can hook up to the system when using the TV-out? (thus driving up the cost of the system even more)
local multiplayer would be more of a console thing, where this would not. actually, the idea of it being a 'hybrid' only goes as far as it connecting to the tv. the games would be made with the same philosophies of their current handheld titles. this would have pokemon gen 6/7, super mario land galaxy, new super mario bros. 3, etc. i'm talking about the actual death of their console line. it would only be a portable console as much as a 3ds is one now. a system like this could probably go for $200 in 2016.
the 'game boy' would have to be more 'casual' and focus on what made nintendo's biggest handhelds hits. there would be less overlap with their traditional line. this is something an nes remix might do well on, for instance.