Be able to set, and override, default bars per line.
I almost almost almost always always ALWAYS want 4 bars per line. If they are overly dense I want two. Nothing else EVER, unless I do something like 5-bar phrases, or very slow (with a lot of measures of whole and half notes). Having to readjust number of bars per line when I add or remove some notes is obnoxious and ridiculous.
Make it so that the score makes sense most of the time. That usually means having a logical default number of bars per line, with overrides as needed by MY choice, not a software "feature".
Things like repeats with multiple endings will come out differently (4 bar phrase with 1-2 extras for 2nd and maybe 3rd endings).
When in doubt, try to make the measure sizes even, and close to the previous lines. IF that doesn't work, try something else but make user overrides intuitive and logical.
Comments
The default number of bars (measures for US readers) per line (system in MuseScore terms) is "as many as will fit". And that is how the vast majority of music is written. But for the minority of cases where a fixed number of bars per system is wanted it is trivially easy to add system breaks at a fixed intervals using [Tools]>[Add/remove system breaks]. See https://musescore.org/en/handbook/3/layout-and-formatting#system-breaks
Format Add/Remove System Breaks defaults to 4 bars per line.
That's a mouse-click, drag, mouse-click or four keystrokes per score: [Alt]-o, DownArrow DownArrow [Enter].
If you regularly score for one instrument or a set of instruments you can create a blank score the way you like it and save it as a template and that becomes your default for new scores.
So, you have your default that you want and a means to override it, already built-in.