I had the same thought when I started making game. But, I choose not to do it, and there were reasons for that. WASD for aiming is worse because you'd need three hands to run, aim and jump at the same time. The best option a mouse, which works the same as in 'Rusted Moss'. I'll consider adding mouse-aiming option. Thank you for your advice.
EDIT : I removed aiming process and let the player shoot directly with WASD. It would be much easier to shoot now.