so, for high-goal offensive, here are some ideas.
with a catapult, we could have a taut net basket to hold the ball. the catapult would start in a raised position and then would be released to launch the ball into the air. fairly accurate, as in it's relatively the same force over and over, so we'd know how much to move around.
with a cannon with a springloaded device, we'd have a chute and a platform inside it. the spring attached to the platform would be loaded (coiled) and then we would release the platform, pushing the ball up and out of the cannon.
with the two wheels, the ball would be launched like something of a pitching machine. the two wheels counter-rotate to launch the ball out. we'd probably need a chute, to ensure accuracy.
the three problems in all these setups:
loading the ball into the start position.
reseting the mechanism - how do you reload the spring? would a motor be strong enough to compress the necessary spring (used in both the catapult and spring loaded cannon)?
releasing the platform or catapult, etc. possibly use a solenoid pin. need some ideas on this, so think about it.