I want them to throw some sort a water-balloon like device at each other and have them try to gently catch it. Except this balloon explodes with fire instead of water if it can't catch it softly.

Capture the flag generally implies "transport the flag."

I was going for something like the economy in StarCraft. You have a stream of poles shot into the arena from catapults, and scoring is based on how many get transported back to each team's goal. (Minus the exponential increase, though.)

I can't read this and not think Quidditch-minus-Snitch.

Quadditch! Man, forget the Robocup (http://www.robocup.org/), I would watch the hell out of a Quadditch tournament.

You could use a little copter as the snitch.

In the eternal words of HPMOR!Harry,

"And then whichever Seeker gets lucky swoops in and grabs the Snitch and makes everyone else's work moot. It's like someone took a real game and grafted on this pointless extra position so that you could be the Most Important Player without needing to really get involved or learn the rest of it. Who was the first Seeker, the King's idiot son who wanted to play Quidditch but couldn't understand the rules?"

I entirely agree that the seeker less variant is a more interesting participant sport. The seeker variant will probably be requested since it is canon, at least at first.

On the other hand, "keeping your eyes moving in a pattern" sounds like a thing that would be interesting for computer vision.

Or "build the biggest tower"?

Build and defend!