Ideas from the mind of koopakicker:
-Reversed weapon damge: Wooden and stone weapons deal more damage while in this active zone while iron and diamond weapons are weaker for an unxpected turning over of tables of battle. Your wooden and stone weapons are temporally renamed to say things such as 'unexpected turn of events' or 'sweet payback. Ie. [player] was killed by [player's] 'unexpected turn of events'/'sweet payback'. Tributes that reckonise the
-Combat frenzy: Everyone in this zone is gradually forced towards each other by walls or teleportaion relative to a zone, that lead them to the centre of the zone and everyone in here recieves swiftness, strength and nightvision buffs. Regeneration and wither effects are also present (overall they should be balancing each other out, so no overall hearth gain/loss and are there together to hide players hearts while in this zone).
-You've been sponsered... to die: Rare and useful items (such as diamonds and golden apples may spawn relatively close to players within this zone while active, and 4 redstone are also spawned (as dust) on the ground to mark the corners of the trap's area around each item drop. As soon as anyone enters the area it will explode after a very short delay.
-Phaze zone: While in this active zone, everyone is telported back to the position they were 5 seconds ago every 30 seconds (with everyone on a random starting number to begin with so noone's phaze is in synch with another tribute), with the overall effect of being to hinder the movement of tributes within this zone, while advanced tributes could use the phazing ability to jump behind a pursuing enemy.
(A similar idea could be after so long each pair of tributes that are closest together switch places within the zone, meaning that the tributes would know where the other person currently is and they'd be drawn in to fight, though this isn't a main idea as tributes forming groups would make this one less special and redundant in this case).