*Copies and pastes from #srb2fun*
I'm sure this has been brought up before, too lazy to check. Anyway: Rear view mirrors. Shouldn't be too hard to implement, just a small screen a la split screen mode in a corner somewhere, which is really just a duplicate of your screen with the camera locked onto your character in chasecam 0. Now, just apply the proper camera angle. Also, there should be an option to choose which corner of the screen the camera would be locked to.
Better yet... Instead of having a locked camera position, make it so that you can set what angle the camera is set to, which could be controlled with a console command like 'alt_camrotate 5'. Or something.