I have been thinking about a project like this for Battle Field 3.
The mouse aims the person and the gun, so that is your (turn right or left) and (look up or down). Those functions could be incorporated into the gun possibly. The trigger would be the fire button (left mouse button). Then you need a button for the right mouse button and an encoder to work as the mouse wheel for weapon selections.
For walking, you almost have to have buttons. Perhaps a pedal or two for your feet to hit?
The biggest problem I can see is that for BF3, you need around 12+ buttons not counting the direction arrows. That many buttons will be difficult if you do not want to use a keyboard.