An analysis of DOTA 2's new patch has revealed a whole host of treats for Halloween including; a new map, a new game mode for that map, as well as tweaks that improve everything from the UI to particle effects.
Most of this comes from Cyborgmatt's excellent breakdown of the latest patch. His analysis has revealed several interesting titbits:
The new map, referred to as "Dire Tire", is a modified version of the existing DOTA2 map and is still a work in progress. It is also believed that a new game mode being made specifically for this map is on its way for Halloween, based on several clues Matt picked up whilst rooting around various files. The clues include images of a tiny Roshan, and candy pumpkins that can interact with said tiny Roshan. References to 'Trick or Treat', and a glowing area within each team's map, and a new animation for creeps also help feed the conspiracy theories.