Layouts
I hope I don't request a feature that already exist within MuseScore or that has been requested earlier.
The idea is that the top level structure of a composition from the first to the last note is a Layout that consist of Sections.
{Layout {Section}}
{Layout {Section 1} {Section 2} ... {Section n}}
{Layout {Section 1} {Section 2} {Section 1} {Section 3} ... {Section n}}
Each Section is a measure block, that is any number of measures that have been bound together. Sections can be freely moved within the Layout, copied etc.
Now this Layout idea might be at odds with the existing Part feature, so some thinking is required.