Lyon, Henry, Harris, Uthappa, Neesham, Abdur Rehman, Fawad Alam, Stokes, and Bravo/Pollard come to mind as players fit and good enough to play in this WC yet have missed out on selection for various reasons.
Quite a few decent Associate players missing too on account of their sides not qualifying.
Perhaps I'll go by each country's squad and try to list out good players who have missed out, + the best of the Associates who aren't around, and we could probably form two decent playing XIs out of that list.