Kinda like the GC and GBA for the few games (4 swords and paceman)
Correct. But with the advent of wireless standards, it doesn't even need to be a piece of Nintendo hardware to work. In the case of something like Jackbox, all you need is a touchscreen and you're good to go. The upcoming My Nintendo mobile apps would then facilitate this.
Could theoretically even funnel in physical buttons if your mobile hardware had a gamepad attached. Which is a lot of layers, for sure, but sounds feasible.
It might be that their online network lets you attach all these things together. The game wouldn't know/care if your friend was attached in the same room or across the continent. My Nintendo attaches you to the game session regardless of its locality (same room, across the world, etc.) or hardware (NX handheld, NX console, iPhone, etc.).I hope it isn't so reliant on me needing to gather friends to make things fun, that online basics and traditional gaming are hurt .