http://upload.wikimedia.org/wikipedia/commons/1/1f/Manicouagan-EO.JPG
To the isle the Imperial City is situated on here:
http://guides.gamepressure.com/theelderscrolls4oblivion/gfx/maps/198696190.jpg
No doubt this isn't news to the fans of TES living in Canada, but while planning some routes for a trip I am taking this summer I stumbled across this awesome lake and island in Quebec that made me immediately think of the isle the Imperial City is situated on in Lake Rumare.
Some pretty good geological similarities, if scale is a little off.
But it just looks like a place that would be IN The Elder Scrolls -- the highest point on the island is called Mount Babel, which sounds like somewhere you'd summon Sheogorath or something.
As a bonus, the island is near inaccessible by mankind, due to the rugged terrain. Not a single paved road exists on the island from what I can tell. It's just cool in so many ways.
Island: http://en.wikipedia.org/wiki/Lake_Manicouagan
Lake: http://en.wikipedia.org/wiki/Lake_Manicouagan