I think a game that successfully blurred the line was Demons Souls and its descendants. You would see the ghostly outlines of players doing the same level as yo

I think a game that successfully blurred the line was Demons Souls and its descendants. You would see the ghostly outlines of players doing the same level as you, you could read and leave messages for other players, and you could summon or invade another persons game. Still despite all this, it had very much the feel of a singleplayer game (and could be played as such).