Simple game idea, but well presented - looks quite professional. Seems like could be fun for two players.
But, obviously the classic pong formula only lasts so long. You could introduce something else, like, say powerups that spawn in the middle and whoever taps them first gets them or, for example, each bounce earns you some points that you can spend, for example, on units that march toward the enemy fortress (like throwing some tower defense in there).
That flag animation is nice and adds some life to the background, you could also add concerned little people popping up every once in a while in the main window or towers - funny AND distracting.