Implement hook in the places sidebar for middle clicks.
So Nautilus can open a new tab when that happens.
Original report:
It should be possible to open one of the folders or bookmarks in the left sidebar into a new tab by holding the ctrl key while clicking.
This is a combination users are used to since browsers behave the same way and while it obviously isn't possible to use the combination inside the main folder/file view, it would be very helpful to have it working for the sidebar as an easy way to open up new tabs. Especially on laptops with touchpads you don't have a middle mouse button available and using context menus to open new tabs is painful and obstructing the workflow.
I very often find myself trying to open a new tab like this on my laptop and end up just using the current tab because it's more convenient than opening a new one.