1. Useful random encounter tables, by environment. I really like the example "Sylvan Forest" table in the DMG, and would like to see a lot more like this, including variants for certain environments (e.g. Forest, Deep Forest, Cold Forest, Sylvan Forest, Haunted Forest...).
2. Battle maps, by environment. When the party gets into a scrap in the forest, I'd like to have a selection of forest maps in which to place the encounter. Not too many -- maybe 3-5 per environment. This would be a very expensive product, since quality map-making does not come cheap.
3. Big list of interesting traps, puzzles, hazards, and obstacles. I have a lot of trouble coming up with fun dungeon puzzles on the fly (and the best traps/hazards/etc. are actually puzzles). It's a huge problem for me because I love improv-style gaming, but I also love a good dungeon crawl, and I find the things that make dungeon crawls entertaining require a lot of forethought.
4. Are you familiar with any of the Savage Worlds Plot Point settings, with their "Savage Tales?" These are like mini-adventures, along the weight of, "You find a treasure map that leads to a crypt hidden in the bend of a river. Six wights, one of which is a leader with maximum hit points and a +2 longsword, guard a treasure containing 3,000 electrum pieces and 4,000 copper pieces." Actually some of the Savage Tales get much more elaborate than that, but usually not more than 2 pages or so. I like the Savage Tales because they are a bit more robust than a plot hook -- they don't just give you the start of a mini-adventure, they give you the whole thing -- but they are small enough that you can just drop them in where-ever you need them.