Say you want to input 'firefox -profilemanager'.

You can type in 'fir' and hit TAB and probably 'firefox' gets completed depending what's installed on your system. Now it would be sweet to key in a dash (-) to indicate you wish to input a switch and then either hit TAB to see all available switches for the command or type in, say 'prof' and then hit TAB to have it complete the wanted switch. (i.e. just like it now does with commands or file names)