Well, you’re the web developer/designer, you don’t have to accept everything at face value.
If you want something more unique, write the custom CSS/JS for it.
Here’s my Bootstrap 3-based site, with a sliding right side-menu. And within each menu item, it slides up or down to open up a sub-panel for submenu items.
So just try something new idea how to make another way.
The truth of the matter is users have been accustomed to some UI/UX design conventions. So your menu either slides from the left, or from the right, or from the top. That’s what users “expect” and know.
If you make the site hard to use due to some unconventional navigation, just for the sake of being different, then you risk losing users.
In design, you want to be different, but not too different. And certain UI/UX conventions, you have to accept.