I find all of the camera angles a compromise in GT and would love some more views/customisation options. I use bumper view purely because it gives the best view of the track and cars around you along with the rear view mirror. However, when following a car closely your view ahead can be obscured quite badly.
Cockpit view has too small a FOV and the windscreen reflections are way over the top. I'd also like the option to remove the steering wheel from this view as in some other games. The roof view is too far up to feel immersive for me and I lose that sense of being at one with the car. The option to have the mirror in every view is a must, it's so much easier to be predictable when driving around others when you can glance up and see where the cars behind are on track.
The game desperately needs the dashboard view found in the likes of Assetto Corsa, this view, placed just inside the windshield still gives you the immersion of the cockpit view, but with a much better perspective of the track of the track around you. Now I've driven with this, its difficult to go back to what GT offers. Another feature that would help hugely with clean online racing is a spotter. This is available in all online races in Project Cars 2 now, at least on the PC version, and I find it incredibly helpful, telling me when i'm clear of a car alongside and on which corner of the car a rival is approaching etc. I'd love to see a similar system implemented in GT in the future.