I've recently gotten into PDFs, and have seen a lot of great stuff, and a lot of eh stuff.
I loved "The Pumpkin Patch" by Patrick Younts. I'd love to see the other beings mentioned in the ad and the introduction, either as a free download or even as a sequel.
In that vein, I'd like more products that add a folkloric feel to D20. Myth, folklore and fairy tales are a rich vein of inspiration for games, but so far, they've mostly been used in non-core games. Groups shouldn't have to play "Grim" to get that feel in their games, especially (as Patrick points out in "The Pumpkin Patch") as it provides a very different, and alien, feel to games, which makes fantasy games strange again, when too often they risk becoming very mundane.
I'd love to see more products that crank up the eeriness of a game, instead of the horror or the gore. Arcane magic, fey and the undead all seem ripe for this. Similar products that up the creepiness -- but not the power -- of the drow, goblins and other they-should-be-creepy creatures would be very appreciated.
I'd also like to see a product on adepts, fleshing them out beyond the bare bones description. Heck, I'd like to see a product on the core NPC classes generally.
I like the existing PDF products on holidays out there. They add a lot of flavor and verisimilitude to a setting without making it not be a fantasy world. I'd love more products in this vein, like foods and drinks (not magical, although those are fun), gods for non-adventurers (very few adventurers would be midwives or worry about the viability of the harvest, but both would be critical to a community) and so on.
Although I can always use more spells, I'd prefer they have a great deal of flavor to them. Player spell-casters can do pretty much anything at this point, especially kill their enemies. If there's going to be another spell in that category (especially combat spells), they need to be COOL. Along those lines, there are several 1E spells that have vanished from D20 -- spells like Chariot of Sustarre, Glasee and Glassteel do things that aren't possible in the SRD, and adding spells like them would be very much appreciated.