EDIT: Also, to the multiplayer, I can imagine a game of semi-volleyball, with different items acting as balls. The player will have the ability to kick them left and right, and they will appear accordingly on the other player's screen. Maybe they can speed up after time.
Players have to both reflect projectiles back and decide when and where to add new items into the fray
That's a good idea! Or a special item (let's say, a scroll) which player takes it (shouldn't kick it) and it will spawn a number of random objects to the other player. I'll look into that idea. Thanks!