Should be easy to implement, hopefully - Copy the map and strip it of skyshards.
I've spent a LOT of time playing solo in Cyrodiil and the Imperial City... The area pretty much already IS a PvE instance. I've literally never dealt with getting killed by another player in Cyro outside of the New Life event. IC's a bit different and the districts tens to be more populated, but the sewers feel the same and 90%of the time I don't see another player at all.