I personally love to drive hood cam, but depending on the route, its sometimes difficult to see far infront. The addition of a roof cam would help by being alot higher up, personally think this would help the enjoyment factor a little.
We need all the cams we can get for maximum accessibility. There could even be an absolute adjustment for total custom camera angle (to a point) for those fine tweaks for sim rigs and the higher-end people would totally eat it up.
I was watching this video the other day:
And he was complaining about the camera angle being too far sunken back for his setup. There should totally be an option in the menu for advanced configurations so that people with special setups can just tweak the camera angle to exactly where it needs to be to make it perfect. This simple fix would open up the Forza series to the higher-end racing sim market definitely.