It's hard for Fortnite to keep any secrets these days with people combing through the game code to find hints at what is coming. As for what is expected in the event this year, it will likely follow the same pattern as Fortnitemares in years past. That will mean a smattering of skins and new cosmetic items, which can be seen in the image below.
Some surprise items are expected to be unveiled when the event kicks off. Parts of the map are expected to change with spooky trimmings and decorations to get a Halloween vibe going around the map. So far, zombies have been seen in some of the new skins and a new emote. In the past, AI-controlled zombies have been part of Fortnitemares events, and it wouldn't be unexpected for them to return.
Face your fears.— Fortnite (@FortniteGame) October 28, 2019
Tomorrow, 9 AM ET. pic.twitter.com/YWHaxuDSPE
Players weren't big fans of the zombies in the past, though Epic Games has worked to make them better to play against. A leak from August also tipped some IT: Chapter 2 crossover items with speculation that some of the items could turn up in Fortnitemares. In recent Fortnite news, Epic sued a former employee for leaking Fortnite Chapter 2 details. The employee is Ronald Sykes, a "User Experience" tester that was able to play the game early.