I really like the trick in this blog: https://www.interworks.com/blog/rrouse/2016/01/04/creating-collapsing-menu-container-tableau
This trick allows you to hide/show elements by placing everything inside a floating layout container and placing it off the screen.
Unfortunately this solution is not very simple and requires advanced knowledge of dashboard actions and layout containers.
My idea is that it would be really great to be able to have layout containers that can be triggered to collapse.
So, when you add a collapsible layout container to a dashboard it adds a button that, when clicked, shows or hides whatever is contained within it.
As an additional feature it would be great to be able to customise the button with a custom shape too.