My brief thoughts:
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A general "Wait!" or "Stop!" signal should be putting your right hand up, palm out, in a general "Wait" sort of motion.
A "Don't do [a thing]!" signal should be done by indicating the thing in the ways mentioned below and then drawing right hand across throat, palm down, in the immortal "This will kill you!" gesture.
Defenses should be indicated by pointing at the defensive wall.
If you want to signal the strength of individual defenses, 1) point at defensive wall with left hand, 2) raise right hand over head with between one and five fingers up (indicating which defense you are referring to), and then either put right hand up all the way for full strength, halfway for half strength, and completely down for damaged. Right hand should be palm down. (I should mention that Defense #1 is always the low bar, just to make things clear here.)
If you want to indicate which defense to attack: 1) Point at defensive wall with left hand, 2) raise right hand over head with between one and five fingers up, and 3) make a fist with right hand ("Charge!") If you indicate strength of defenses and then immediately make a fist, it should be interpreted as you wanting to attack that defense on account of its strength. If the robot is misaligned, align it with the instructions provided later.
Actions pertaining to the tower are indicated by pointing at the tower.
"Shoot into the high goal" or "You're aligned for the high goal" is indicated by pointing at tower with left hand and making a "basketball shot" motion with right hand.
"Shoot into the low goal" or "You're aligned for the low goal" is indicated by pointing at tower with left hand and making a "bowling-ball-roll" motion with right hand.
"The enemy's defense has been breached!" is indicated by pointing at the tower with left hand, raising right hand straight up with all fingers extended, and then making a motion as if you're hitting a buzzer open-palmed at slightly above eye level (basically, just rotate elbow down).
"Surround the castle!" is indicated by pointing at tower with left hand and making a fist with right hand.
"Climb the tower!" or "You're aligned to climb the tower!" is indicated by pointing at the tower with left hand and pointing at the sky with right hand.
If the robot is misaligned, align it with the instructions below.
"A boulder is here!" is indicated by pointing to the location of boulder with left hand and then using both hands to mime "holding a boulder" over one's head with hands on either side of the "boulder." (Repeat as needed.)
"Pick up the boulder!" is indicated by pointing to the boulder with your left hand and making a motion as if you're picking up a boulder off the ground with right hand (without stooping).
"A defensive robot is here!" is indicated by pointing to the defensive robot with your left hand and putting your right hand over your heart while making a fist. If the robot is moving, follow it with your finger, bringing your arm closer towards you if it's close to you and further away if it's near the other side of the field.
Directing where the robot moves is done as follows:
Extend both arms straight out in front of you if you want the robot to go in that direction quickly.
The same could be said with extending both arms to the left, to the right, straight back, or at any other angle. (For straight back, put arms up over head, bent at the elbows. You know what I mean.)
If you want the robot to go more slowly, only extend arms partway in said direction.
To stop, use "stop" signal above.
Finally, if your robot or any other robot is broken in some way, point to robot and then put both palms on your forehead.
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That's really all I could come up with for now. I'm sure there are plenty of other signs that need inventing, though.
...and I guess I don't have time to do any more of this, so why don't you invent other signs yourself?
-Thromgord