Messaging is sort of a given, but as a device that absolutely must be child friendly and safe (despite the Giant: WARNING - 3D FEATURE ONLY FOR CHILDREN 7 AND OVER) I don't think Nintendo can do away with Friend Codes. The prospect of being able to randomly message anyone off the street simply because they're carrying a 3DS with StreetPass enabled wouldn't exactly make it "child safe."
The thing is, Nintendo already has built in parental safety features so the codes wouldn't even be necessary if every parent who bought a 3DS for their kid bothered to learn enough about the device to set itup and implement those features. Maybe there'd be less of those "child buys over $1,000 worth of software through AppStore" stories if they did.
Depending on how the browser functions, that could open up more potential features, so the 3DS is pretty flexible between that and future firmware updates.
Things like temporary game pack sharing for multiplayer sessions would appeal to me more, but I still see the appeal of being able to walk around collecting other player's Miis and using play coins to unlock more features.