Here's my brainstorm:
Void - Random blocks in the segment randomly start to disappear, revealing the void.
Storm - Rain & lightning, like a storm.
Raining TNT - I don't know if this would cause lag, but TNT would rain from the sky in the certain segment.
Zombie Hoard - A hoard of zombies would spawn on you in the segment. #WalkingDead2014
Arrow rain - Arrows rain from the sky (from monkeys? aha...)
Witch spawning - These guys are mean. lol.
[A goofy one] The Chads - Chadthedj clones spawn everywhere that toss harmful potions everywhere.
Also, if this is even possible (which I think it is in 1.8) a spinning cornucopia would be neat.
I will edit more ideas on to this list as I think of them.
EDIT:
Arrow storm the skies rain arrows on the segment
Omg, lol. We had the same idea. xP