The left nav is right out of the box already out of space for adding custom pins. As you'll see in the screenshot below, none of my added pins show up on the left nav, I have to select the "All Navigation Items" icon to see them. I think the icons marked in orange below could definitely be moved into the "features" or "all navigation items" icon pins and would also love the ability to control for myself and others what icons appear on the left nav.
