Basically make it as easy as Mario Kart Wii, so fast random and lobby connections.
I'm absolutely very happy with the Wii Speak only for friends option. I stopped using my 360 headphone because of all the idiots on the other side of the connection.
The option to add anyone in a lobby to your Conduit FC list right away would be very nice (iirc that's also in MK).
But basically no lag + easy connection is everything I really want, the rest is bonus.
Thats nice that you dont like hearing other people online but all you would need for that is a mute function. Stopping people from communicating with their teammates just because you dont know them is lame and if you dont like to hear them mute them. Its lame Nintendo is enforcing unneccessary limits.
Also I love the spectator idea. It was in SSBB but they completely messed up by not letting you spectate your own friends matches! Also I to would love to have the flexibility to switch the game options so its 1 vs 15 if you really wanted to. Thats the sort of streamlined experience it needs to be.