We do plan to make the sidebar more extensible for plugins and theme components, but aren’t aiming for full backward compatibility with things that had targeted the existing hamburger. It’s a little too early for us to dive deep into this ourselves, but we’ll kick off a discussion about it at some point in the not too distant future. (If there are enough folks who want to talk more about it now, that’s fine too - feel free to start a topic and we’ll join in later).
Separately, we do plan to add some built in ability for folks to add custom links themselves. I’ve started a topic that outlines our initial plans for that feature here: Ability to add a custom section of links to the sidebar