What a enjoyable experience, the polish and beautiful presentation was an important factor in making this idea work. And it works really well! I have some nitpicks and suggestions, but that's in the context of this already being a good game made by someone obviously skilled and not criticism on something you did "wrong":
- Trunks should be redesigned. They're too friendly and inviting, and the long "orangutan" arm being hurt by what looks like the perfect climbing trees doesn't make intuitive sense to me. Thorns would maybe be too on the nose, but maybe you can do something artistically in the way of colors or angles to make them slightly more intimidating?
- The arm shader could benefit from feeling/looking even more satisfying. The base ("shoulder") doesn't scale with the first segments of the arm, which I assume is a bug, but it also highlights how the gradient of the arm doesn't quite match the body. It's too dark on the bottom, it has a bit too much depth compared to the character.
- Another thing that could help the arm is if it had a slight inverse kinematic to it. This would maybe alter the gameplay a tiny bit, but I'm talking mostly cosmetically. This is achieved by having a "stiffness" on each segment that scales with their thickness, the base will be almost stationary when the arm is at half length (e.g.). Like a branch growing. Here's an exaggerated example from a shader I wrote a few years ago (YouTube), notice how it stops responding to the IK chain as the thickness grows. Something like this, only way less flexible, just to give the arm a more "physical" feeling while still keeping the addictive "draw a line" game play.
- Additional type(s) of tree requiring a different kind of movement and priority, just to add another bend in the game loop, perhaps some that's targeted towards being made easier by certain upgrades. No need to add too much noise to the mix, the recipe works, so it has to really make sense for you to put it in beyond just "more content".
- Add some random initial pitch to the arm stretching sound as well, like a 10-20% range or something, so every pull has a slightly different sound range to it. That's the trick birds use to not sound annoying!