Multi-monitor gaming systems are becoming relatively popular right now. Adding support for these systems is rather easy to do by (letting the user) limit the HUD and menu screens to a certain width/height and add an X/Y offset to any HUD/menu elements on the screen.

Additionally: FOV in-game should either be calculated using the vertical component (recommended!) or the user settable FOV shouldn't be limited to what is considered 'sane' for single-monitor setups! Right now the ultra-wide crowd is still left out FOV-wise.