Setup. Two throwers 12 to 15 metres apart with a 5 metre channel between them. One keeper in the channel.
How it runs. The throwers lob the ball to each other over the keeper in the middle. The ball has to be catchable: no higher than the middle keeper could reach with a good jump, and it must land inside the far thrower's area. The middle keeper comes off one leg and takes it at the top. Catch it and you swap with whoever threw it. A thrower who drops a catch goes in the middle too.
Make it harder. Throwers may volley it from their hands. Then two keepers in the middle compete for the same ball.
With 4 keepers. Two channels: one with a middle keeper, one pair just throwing and catching, then rotate.