I don't know, just throwing ideas out.
Let's say you use Graves' Q, and then use it again after the 12 seconds (+2 bullets) you use it again.
Now, if you level up to max with his Q, then the wait would be 8 seconds, but you still might wait 10 seconds before firing again so +2.
I also see that a free ultimate is horrible. Making it cost half the bullets will mean you can fire it and then have 5 left to still do another move.
Now, I think it might be nice if at level 10, the time to regain 1 bullet would go down to 4 seconds but that's not up to me.
It will allow for a max Q to regain 2 bullets as it finishes its cool down.
No problem with W at all.
It might buff his E.
And it does nothing to Graves' R.
For Miss Fortune, it would also be a buff to her W, might be bad for her Q, and might be bad for her E, so it could be tweaked for her to regain from 1-6; 1 bullet ever 5 seconds, from 7-12; 1 bullet every 4 seconds, and 13-18; 1 bullet every 3 seconds.
This might buff her W by a lot but it is also a hinderance to her Q but it makes her E less of a hassle.
And for Tristana, the fact that her Q is already free means that its fine to remain free.
This might be a huge buff to Tristana. Even with cool down reductions, with the cost of 2 bullets per ability, she will regain all bullets lost by the next use. She can remain at 1 bullet per 5 seconds and it would still be a buff.