[Randomness]

I like a bunch of the ideas on this thread, so I combined most of them: varied game pieces/roles, open field, durable pieces, tetras, and an easily understandable game.

Here is my idea:
Have a variety of medium-sized PVC game elements - some a simple pipe with a cap on either end, some tetras, maybe some other weird shapes, with an average dimension of 2' x 2' x 2'. This would make it very difficult to manipulate each one with the same grabber, but also allow a simple box bot/snow plow (like the robot from the game animation) to score. Each of these elements needs to be brought onto your side of the field for points, similar to a capture-the-flag or Hungry Hungry Hippos. Last, PVC is durable and easy to find (although making tetras would be hard).

The endgame could revolve around stacking the PVC game pieces - give 5 points for each foot off the ground, if fully supported by other pieces and not a robot. In order to prevent a team from knocking over an intricate stack, there should be a protected zone large enough to fit a stack or two, but small enough that you couldn't pile all of the game elements in (points are still scored as long as the game piece is on your side of the field.) Stacking PVC elements on top of other elements, and not on a robot, would prevent dangerous 10+ ft-tall towers that could topple. Smaller towers would not be a major issue - a 2' PVC cube is not a significant safety hazard if elements are removed from the top down, and stacks will not tumble a significant distance out of the field - especially if the protected zone is in the middle of the field.

One problem is that announcing the score will take a while (although it does add a bit to the suspense)