I've been looking for a link to post a 'pie in the sky' thought and twq's question on cross town travel gets near: http://citycyclingedinburgh.info/bbpress/topic.php?id=15352#post-196991
If the Scotland St tunnel were to be opened as far as St Andrew Sq, (I know it's blocked further on) and a large lift were installed to get people and bikes between the tunnel and the square, it would have so many benefits linking the centre of town for pedestrians and cyclists with a huge swathe of N Edinburgh and Leith.
With some careful thought about George Street and the new St James quarter, there could be onward links to the west end, and south to the Bridges.
I can't see a downside, and all recent reports say the tunnel is sound. Presumably it could be brought into use like Rodney St tunnel was.
Even Essential Edinburgh must see how opening up their area to convenient access for active travel from the north would be a good thing.