That's was exactly my final assessment of the game as well lol. It's interesting in concept and challenge but once you "got it" the game is done. I was considering in make a "wave mode" where planes adopted different "strategies" like formation flying or focus in " low level bombing" but still fells the same.
Probably I need something like power ups , events and different "maps" to make it more interesting, but I would never finished in time lol.
Thanks for the feedback :D