Why not implement something like this into the game? When you pull an arrow back, you have a period of time when your aim steadies, you zoom in (If your level permits it) but if you try to hold the arrow for too long your arm starts to shake and your aim deteriorates, until you finally have to lower the bow and try again. (no cooldown or anything, that would be pushing it)
As you level up, you will be able to use more powerful bows and be able to hold the arrow in place for longer. When you start off, it could be either 20 seconds (actually quite a long time) with a weak bow, or less time with a more powerful bow, this would give you a choice (trade off speed and accuracy for more power). As your 'marksman/strength' increases, you will be able to zoom in, use more powerful bows, have a steadier aim and draw the arrows back for a longer time.
I think this would add a whole new dimension to the archery aspect of the game. It would make it more realistic and more tactical because you would have to time the perfect shot.
A small 'grunt' or 'sigh' from your character could indicate that he/she can't hold the arrow any longer.
This was actually done very well in the game 'Thief: The Dark Project', a classic