Currently all notebooks/items in the sidebar have the same color and no icon, make it hard to skim through them.
Ideally you could set a color for each notebook name and all it's children would inherit that color. So, for example, you could change the text of those items to red, or add a red indicator before them:
The implementation could be similar, but the idea is different.
I don't care as much for the indentation/nesting level as the grouping of notebooks to make it easier to find the notebook you are looking for.
I want for example to have all personal-related notebooks to be green, all notebooks related to business be blue and all gaming-related notebooks to be orange. (just an example, not my actual use case).
Another example is how Onenote shows tabs at the top AND each top has a different color.
If all the tabs were the same color it would have been a lot harder to find the one you need.