In the Waterlogged update post the devs have stated that: "We also have a new special thing we have never done before that we're pretty excited about for October that is super secret.", so obviously here's a thread where we can guess what they might have meant.
Tbh, after looking back at RoT, I no longer know what to expect from Klei. Maybe they'll rework summer to make it a fun season. Maybe they'll add something for the multiplayer experience such as a voice chat. Perhaps they'll add progression and randomness to the game. Or maybe they had been working for a few months now on a world generation/biome overhaul update before they released the Waterlogged update.
October may be the Spooky Month but I doubt that the teased content will have anything to do with the Hallowed Nights.
What do y'all think?