I'll go first!
A basic beastiary with various different common monster stats, or a themed one, with blocks and descriptions for foes of a certain type, like undead monsters, or demons.
An exploration module with rules for overland travel, making camp, discovering things in the wilderness, etc.
Stuff for generating dungeons!
Supplements for steadings like towns or cities! Maybe this might be linked to exploration!
A big book of items! Weapons that do special things! Crafting rules!