In case you're not aware, if you hang out on a zipline during a wave then you're basically immune to ground bugs. So how do the bugs deal with you? They shoot you with projectiles.

If you were ever able to move fast enough on a zipline that you could reliably dodge these attacks you would, essentially, become invincible on a zipline. Nothing short of a Bulk Detonator could stop you from just hanging on a zipline the entire wave and shooting.

Please keep this in mind if you feel like the zipline needs a buff, which it doesn't. But if you really feel like it does then it would have to be something not related to uphill speed.

Edit : There's been some great discussion and suggestions about the zipline in this thread and I just wanted to highlight 2 of those ideas.

One is to replace flat ammo count with a total length for zipline. Each zipline subtracts its length from the total while still having a minimum and maximum length. This makes short ziplines way more viable to use.

The second is to allow you to carry your momentum up or down the zipline but removes the ability to change directions. This could work as a mod (or OC if they ever decide to add traversal OCs) and I think it gives you a lot of cool options while still having a legitimate drawback that also fixes the camping issue.