Okay straight to the point.
What's staying?
The overall lore of the game, being two stories (Drake Randy working at Jolly's in 1985; Josh Randy exploring the same location albeit abandoned in 2018)
Most of the animatronics.
The atmosphere
What's being added?
New and detailed cutscenes and storytelling
Custom night for both modes
New characters for 2018 MODE (the rest of the monsters)
Heavily improved coding and AI mostly thanks to the game using UJN's code.
What's being removed?
Everything relating to the chaos spirits just like with the previous 2 games.
The Fusion Illusions
Amalgam
The 1985 Repair Minigames
Fazbear Challenge
Prototype Jolly
Classic Tweetie and Maxie (will be replaced with similar characters)
That's all.