why should invent a bycicle?
everything is generally invented. just need to make it usefull and easy to customize and understand,
of course it should have f-number. exposure control, shutter speed and ISO and custom white balance.
and in viewport it needs to be showing planes of DOF.
and, yes, some unique and recognizable form of the icon of camera showing in viewport. may be smthng like digital cams
and,yeah, focus selection in viewport could be very useful:)