I had this idea for a mod since I have been having issues with the command syntax after all the new things being added. The idea is that when you hit a keybind (set in setting) such as CTRL while typing in a command, it will display the proper syntax of that command in the chat line so you can type of that syntax to make sure you are doing it correctly.
example: I am typing /give and then hit CTRL and in the chat line it keeps my /give that is already typed but adds in a lesser opacity <player> <item> [amount] [data] [dataTag] that way I know I won't mix up the item and player or something like that.
I'm unsure whether or not this functionality could be added to the command block line as well but if so, I think that would be very helpful.
I think this should be possible and will be very helpful in learning the new commands that will be added in 1.8