Lava Pit: On the fourth wave the pit will open and provide extra danger
Hand Of Fate: If you fly close to the pit past wave 7, a hand will try to grab you and pull you in.
Dactly: Sometimes you'll see a Dactyl. Jab it in the head to kill it for a bonus, otherwise staw away from it.
Pedigree Ostringos: The red Ostringos have two hitpoints, as it were. Hit one once and it will turn into a regular green one.
Eggs: Sometimes knocking a Joust0r from an ostringo causes it to lay an egg which can be collected for bonus by anyone, even non-players.
Optional 2 Player mode: Kill eachother for multiplier
Bonus wave: In every 10th wave you will be immortal and score is for the taking of whoever can get it first. Admittedly this is fairly pointless in 1 player mode, but still makes a change.
Wind: Once in the second episode there will be the occasional wind that effects players, foes, and everything else that isnt part of the arena.
Meteors: Or stray, airborne volcanic rocks, you decide. They fall from above past the second episode. Not that often, though.
Arenas: More than one background, and it's about time