All of the ideas people have had for what it might be seem unlikely to me.
Camera - We didn't see a camera in any of the leaked renders or the official reveal. Companies are designing cases based on those renders which means they'd be designing cases that block the camera (if there was one).
There also just isn't
much point in having a camera. The IR camera on the right JoyCon was barely used, the cameras on the DSi and 3DS were barely used, and even when they were used, it was not something that was a necessary feature or in any way made the games better.
The camera on the Wii U gamepad got used the most, and that was only to take snapshots of your face to show as your player icon in game, hardly an important feature. Nintendo have tried that gimmick, multiple times, and it didn't work out. Why would they bring it back now?
I even stopped using the AR feature in Pokemon Go because it's a gimmicky pointless feature that wastes battery and doesn't make the game better.
But also,
why would the camera need a dedicated button? The 3DS didn't have one, modern smartphones don't even have one, we're still able to use them fine. No matter which way you look at it it just wouldn't make any sense.
Chat - Again,
why would this need a dedicated button?
It's
not push to talk, because you'd be taking your finger off the buttons and right stick mid game to press it. That would be TERRIBLE for gaming. And does not warrant a dedicated button considering only a small portion of games would use voice chat, especially when it could simply be mapped to one of the more easily accessible buttons (probably Z/L/ZL/ZR) in games that actually utilize it. If they actually did this, this would not be a logical place to put the button.
It's
not text chat, because you'd need to navigate menus to select a user and type a message anyway, having a dedicated button to bring up the chat wouldn't save you much time and hardly seems like it would be a priority.
The
one chat related thing it could be (and this would also explain why the "C" is mysteriously absent from the reveal) is a dedicated mic mute button. It would make sense with it being out of the way so you don't accidentally press it. Obviously, there is no "C" in mute, so it might have been used to vaguely refer to "Chat" so as to not make its purpose obvious but will be changed to a mic icon for the final unit. That would make sense, if we assume that Nintendo has any intention to officially support voice chat.
Cast - This is a wild theory overall.
It
could be used to emulate DS and 3DS games by using the TV as the top screen (as some have suggested) and give the Switch 2 unparalleled backwards compatibility. But multi generational backwards compatibility is not something Nintendo has ever cared about, even the Wii U which was easily capable of running GameCube games was (officially) limited to Wii, and Nintendo even had full GBA BC on the 3DS which they never used for commercial purposses even though they could've. They seem to prefer releasing remasters for full retail price instead. I don't see what they would really be gaining from DS/3DS emulation. Customers would love it, but it isn't helping Nintendo's bottom line. And Virtual Console, as we all know, is dead and unlikely to make a comeback. There's more money in remasters and re-releases.
Casting
could also enable dual screen functionality in numerous games. But this is something Nintendo already tried with the Wii U and it was a failed gimmick. It seems highly unlikely they would try it again. People don't want to be required to look away from the TV screen mid game.
An
interesting use case
(but still unlikely due to the complexity of it) would be for the dock to have the ability to connect multiple consoles. As we know, the Wii U had the capability to connect 2 gamepads for multiplayer games. Because the Wii U overall failed, that feature was never utilized.
The ability to
cast multiple Switches to one dock
would increase the cost and complexity of the dock, but could genuinely lead to some interesting use cases.
The obvious use is for everyone to be able to play say Mario Kart or Smash Bros each on their own Switch, using their own save data, but sharing the big screen rather than looking at their individual screens.
I find playing multiplayer on the TV to be more fun and social than playing on individual screens, so this would be an obvious improvement to the experience for me personally as then everybody would still make progress on their save file (unlocking things and what have you) while still being able to play on the TV. It is hardly what I would consider a "genuinely interesting use case" as it's just a "nice to have" rather than something game changing. But it is a
neat additional use case if they already had other uses for the feature anyway.
The more
genuinely interesting aspect that this feature would enable, obviously, is asynchronous gameplay where shared info (such as a map and player stats) can be shown on the TV and each player has an individual screen with private information. Not exactly like how the Wii U did it, but not that dissimilar either, since on the Wii U there was always at least one player playing directly on the TV screen, and only one (but in theory up to 2) player using the gamepad. I could see it being tons of fun in Mario Party, with certain minigames requiring the use of individual screens so players can't see eachother. But that would require everyone to have their own Switch 2 to be able to play those minigames which would a lot of the time lock people out of playing them, making it a
niche use case. So while it would be cool, it doesn't seem likely there would be many games built around this functionality and it would always remain an optional feature in a small amount of games rather than something widely utilized.
That would be almost exactly what certain games on the GameCube did with GBAs, but far more capable since GC-GBA linkage didn't utilize casting (and was far too slow for it) so the GBAs were limited to running simple 2D graphics.
I would personally
love to see multi casting even knowing it would go underutilized. There is enough potential for great times there, I had a great time with Nintendoland and that was just one game and a single gamepad screen, I would love more of that. It's a decent improvement on a gimmick that I wished would have been utilized more, but it is still a repeat of a failed gimmick,
and one that would increase the cost of every Switch 2 due to the more complicated dock hardware, so it is unlikely to happen.
Edit: Come to think of it, multicasting wouldn't require any extra hardware in the dock compared to single casting because one player could simply be acting as the host, rendering the shared screen using the data it already knows. So this could actually be fairly cheaply accomplished although it would drop performance for that one player in games like Mario Kart since it would have to render all 4 screens instead of just the one.
My personal guess is that "C" stands for "Controllers" and it simply takes you to the controller selection screen or allows you to switch the controller orientation/mode on the fly. Anybody else like me who was annoyed by how often they had to navigate to the controller selection screen mid game to change stuff will understand why this is a likely feature. It was a significant friction point for me due to how many ways a JoyCon pair can be connected (dual vertical, single vertical, single horizontal) and often needing to change it between games or even between different game modes (looking at you, Wario Ware Move It and Mario Party Jamboree) as well as needing to disconnect and reconnect every controller and do the L+R dance if even one player left or joined. No other console has had this problem, it's a Switch unique problem, and it makes sense they would want to solve it. It might be boring, but the simplest explanation is often the correct one, Occam's Razor and all that.
Edit2: Added some formatting to try to make the post a bit more readable.
There’s no way it will be casting video the input lag would be atrocious
I cast my quest 3 to my tv and it’s like 2 seconds behind
Casting Pokémon from my iPad also has like a 1-2 second delay
The Wii U gamepad worked by streaming a video feed from the console and had no perceivable input lag (at least to me), it's not out of the question that the dock might have low latency wireless and video decoding hardware to make this possible. Even the Quest 3 which you mentioned works the same way when playing PCVR and I can't perceive any input lag (though I know some people can as VR is much more sensitive to input lag). Of course chromecast would likely be a bad experience because it's not designed for low latency but casting doesn't automatically mean chromecast.
