If I could choose the theme for the next pack, I'd have to go with Mack-styled sprites (characters, animals, and monsters). Why? It's because it's quite popular, even more so than then XP styled ones.
At 48x32 pixels, they're the perfect height (not tiny like VX/Ace at 32x32 pixels and not overly large like XP at 64x32 pixels).
They're quite often requested, quite often supplied as resources, but they're not very consistent in style. These could be made to be either Mack, DS, or RTP styled - it's the height and width that really matters, so long as the style is consistent for the entire pack.