I'd add some doubles, mostly in the .75 - 3 camalot range (though not necessarily camalots. Maybe a link cam or two and some metolius cams in that range. I also like to have doubles of small cams like yellow and orange metolius.

To specifically know which pieces to buy, I'd look at which pieces you place most, and what size the cracks are when you find you really wish you had a piece, but already placed it.