Well in Demon's Souls you are constantly online, but normally this doesn't affect you very much. You'll see hints and messages written on the ground, you can watch other people's deaths by clicking on bloodstains, and you'll see ghosts of other players as they play shift in and out of existence but you are still in a single player experience. Sometimes though you can be attacked by an invading player, or summon them to help you by clicking on signs they leave on the ground. There is also a boss that uses the online aspect very creatively.
I think this general philosophy could be adapted into a Zelda with some pretty cool results.
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