What would happen if ..?

The ball would accelerate until it reached its terminal velocity, where the resistance of the air counteracts the pull of gravity. As it fell further towards the centre the gravitational pull would get weaker, so the ball would start to slow down.

It would also be bouncing off the walls as it fell, even if you dropped it perfectly vertically, because the Earth is rotating as the ball is falling, so the path the ball has to follow is not straight.

It would pass the centre of the Earth where gravity is zero, then it would start to decelerate, eventually come to a stop, then fall back again toward the centre. It would oscillate back and forth, for days probably, until air resistance and collisions with the walls bring it to a stop at the centre.

If you simplify things by drilling the hole from North Pole to South Pole (no rotation to worry about) and empty the tube of air (no air resistance) and we also ignore the gravitational pull of the Sun and Moon, then the ball will continue to accelerate until it passes the centre of Earth. Then it will start to decelerate. It will fall (rise actually) straight through to the other side, pop out of the hole, then start to fall back again. It will go on doing that forever.

Supposing the hole could be maintained all the way through the Earth without melting or being crushed, the cannon ball would indeed end up suspended at the center of the Earth's gravity well, some 12,000 miles deep!

I assume the cannonball would be destroyed at Earth's molten core. The likelihood of that hole being successfully built is not very good. Though I wonder (theoretically) if it were possible to build the hole and relieve the pressure from the Earth's center. Maybe that'd change what happened to the cannonball. if the center of the earth were hollow I wonder if gravity would suspend the cannonball in the center of Earth's sphere.