Here are a few of my theories:
This has been speculated to be possible for awhile. I think that it might be possible, but only for a few select games and Virtual Console. There are a few problems with having full controller support - Rumored two analog sticks, 2 bumpers, larger and higher-res screen... It's virtually impossible. But if you look at a pre-gamecube controller... There should be no problem playing those with your 3DS.
Also, we might be able to use the 3DS for some more touch-based games. A 'Cafe' series, similar to the 'Wii' series, maybe. The chances Nintendo would do this are higher, as Nintendo would definitely want a way for people to not have to buy more controllers. And what better way then to get people to bring their 3DS?
Another thing, if the 3DS is a compatible controller, then we might be able to find out something about the cafe controller... Look at the back of your 3DS right now! What do you see? An IR port? Could this be how Nintendo implements pointer controls into the Cafe? It ha been rumored to double as a sensor bar... So it makes sense.
Now I'm not suggesting that we go and carry our Cafe's to the local Bar, that would be silly. Tim Horton's? Maybe. What's Tim Horton's? Canadian Coffee shop. Good place...
On topic, one cool thing that I would like to see in the Cafe-3DS connectivity is Street Pass support. Think about it- You go all day about your business playing on your 3DS... When BAM! You got a streetpass. You quickly open up your 3D gaming device and get a new puzzle piece, go play some Mii Quest... 30 seconds later, you're
This could also link in to a new sort of account system... You can link one or two 3DS' to your Cafe (The account, not the console) and whenever those 3DS devices are near, it finds out all the new streetpass data.
This might be wishful thinking, but I could see Nintendo doing it
3) Game Connectivity
This one's a given... Who isn't going to want to play Pokemon against their friends 3DS', or travel to their friend's Animal Crossing Cafe's town? It's happened since the gamecube, and it isn't going to stop. And I don't want it to
Well, those are just some of my theories. Does anyone else have any ideas?
(Also, sorry if the above is a bit jumbled... I got distracted by someone halfway through writing, so I had to re-find my train of thought >.<)