1.) Giant mushrooms (or even flowers of some sort) come "alive" and hurt players if they get to close.
2.) The spectators become visible, confusing players who is in the game, and who isn't. They might even be able to still have all of their equipment (while not being able to harm the players). It would also be cool if maybe they would appear for a few seconds, then disappear for another few, causing them to appear as ghosts.
3.) Snow and cold air could close in on the section (like the poison fog in the movie) and anyone that touches the snow, dies.
4.) You code a mob to look like Chad's cat and he chases you. When he hits you, he takes away half of the players current hearts.
5.) It appears everything in the player's inventory isn't there, everybody can see what they have (for example, their armor and what they have in their hand), but the actual player can't see it. You would be able to see other player's things though, and it would only be in that section.
6.) Poppies spawn all over and if you touch one, you take damage. They would create a little maze though, where if you jump or walk to the correct spots (maybe they are empty or are fakes) you don't take damage. The poppies would represent President Snow's obsession with blood roses.
7.) The players in FOV in the section becomes very limited as if they are looking out of a jock-o-lantern, making PvP even harder.
I may add more later, but for now, these are my best thoughts.