Okay first off, zip lines sound awesome if you update this with them I'm definitely coming back cause that sounds insane. As for the wedging yourself between plants. I can definitely see that. Though something that could possibly help that I did just think of is maybe adding some sort of... "vertical" tower that could counter the addition of "bomber" drones or something. Which could lead to the plant variants being helpful, while also balancing them to only helping against one drone type.
Also, I don't know if I mentioned it in my first comment, but there was a bug I found where you could endlessly stack plants on top of each other. I don't think that's super balanced, but as I'm writing this I do think it could be an interesting upgrade mechanic. Assuming you add some sort of auto aim that doesn't super conflict with the placing system.
To end off before I write another wall of text, it's always awesome to get a paragraph in response to a paragraph.
seriously though, if you add those ziplines I'm gonna be checking