Fantastic idea! this is brilliant! What about instead of making different balloons tuned for different payloads and planets, you implemented a way to change the amount of helium (or Kerbal counterpart). With this you would have to tune your balloon to your payload. As of now with a tiny payload the balloon shoots up at absurd speeds. if you set just the correct amount of helium in your balloon it would lift far more realistically and give a more realistic trade off between payload and lifting capacity/max altitude. this would also allow you to make balloons that would essentially hover at a certain altitude with out popping like a real weather balloon.