We already do (and have done for a few years now)! Two or more designers in the Mac app can work together on the same document in real-time. Better still, if you invite someone to edit your document from outside your Workspace and they already pay for Sketch elsewhere, we don’t charge you any extra for them. What we don’t offer is collaboration between designers across different platforms — we’ve explained in other answers why we won’t be building a Windows app or bringing our full editor experience to the web app.