Ideas for new levels
As ideas for your levels, you could find some ideas in the levels of Angry Birds:
http://images.thisisxbox.com/2012/07/angrybirds3.jpg
In some level (maybe in a serie), you can do like Angry Birds do: only 1 player at a time on the screen.
When it's the turn of the second player, you show his view.
You can also check the levels of Worms.
https://i.ytimg.com/vi/ggZEa-JwaQ0/maxresdefault.jpg
You can also in some level allow the player to move her/his catapult in the half part of the screen and maybe in handicap matches to move the position of the catapult of the other player
Handicap matches could be great (some themes), think about it.
Add more difficulties, variations...
Some weapons: catapult with 2, 3 appels... but could be limited in 10 games.
Catapult on a moving boat...
If you create a simple generator of levels (with the elements of the game), people can create levels for you, share them on your site, import them like Sokoban game, and you can include some good levels in the game (rated by players). Think at Little big planet (playstation).
You have a good game, go go go