I would love to see the sidebar collapsible but only leaving the rail, as well as collapsing the rail too.
But one thing I miss from the really early release I experienced was the ‘Starred’ mode where I could pick and mix and have my own mode essentially, combining Portfolios, Projects and Teams (which would address some of the issues people are raising above) and also the unique ability to star My tasks which is missing in other modes but Work. Too bad that’s not in this release…
Let’s add a ‘Starred’ mode! 
PS: You can vote for this here: Add a ⭐ 'Starred’ mode to the new sidebar redesign (2025)