I'd like to see the comprehensive trap creation rules that got cut from DMG 1.
I'd also like to see an Unearthed Arcana like section of variant rules systems and house rules.
Also want lengthy section on rules for PCs ruling kingdoms, or guilds. Essentially resource management on a large scale.
Also rules for largescale warfare. The largescale warfare should be somewhat abstract maybe done up like some sort of massive multi-layered skill challenge where each success accumulates victory points which determine the flow of battle. I also liked the flowcharts and stuff from Heroes of Battle.