I find the special treatment of a single window in NotePlan a constant impediment to my work, and really wish it worked more like any regular multi-window app, an arbitrary number of windows and no special "main" window. The inability to use the command bar, or show today/yesterday/tomorrow, to navigate to other notes, or to show/hide sidebars, in any but the one blessed main window (which without sidebars is impossible to visually distinguish from other windows) constantly leads to me trying to take actions and failing with an alert sound. Whenever I'm working with more than one note (very common with today + any non-daily notes) it adds lots of cognitive overhead to the process of ever trying to rearrange my workspace or navigate.
(Things and Craft both work much more smoothly with an arbitrary number of equally first-class windows, and are worth looking at as models here.)
From that foundation, I'd love to see this extended to support the modern macOS model of tabs, where a single physical window can host multiple logical windows. Switching quickly and maintaining a mental context without having to go through the command bar is very useful.
Finally, to fit in with this, the command bar should include an option to open the selected note in a new window rather than always only the main window (or, in the world of multiple first-class windows I'm advocating here, the frontmost window) or a new tab.