This is already established in the beautiful Ursa Custom Theme. There are a couple of problems with that implementation though. I think Obsidian would benefit from giving the option of focused writing in one note. Being not an outliner allows for more long-form writing and it would be nice to make that use case more pleasant.
Here is a comparison between how Bear and Obsidian handle long-form text.
On a more core features level, I think the whole app can benefit from a bit more whitespace when taking notes. Even Roam gives more space on the screen when having only a single note open:
Edit: I am not sure whether this is more of a Plugin request for a ‘Mode’ to toggle or a fundamental Feature Request since Obsidian could benefit from some more Whitespace overall, as the comparison above shows. Unless it is a deliberate feature and I missing something (or my taste in design is just a bit off).