Actually this idea has been around for years, and sadly isn't new at all. I just checked and there is one that's been around since at least 2006, http://da-archive.com/index.php?showtopic=42405
Right, true. Collaborative drawing was basically the "hello world" of real-time platforms back in the day (Firebase, Parse etc). But I think most of those were ephemeral canvases.
Yeah, collaborative drawing is kind of old hat, but when you can use the context of the modern, social web to provide some new modes of interaction around it, it can be interesting again. Same applies to more mundane things like text.