I bring this up as I was working on a piece in 6/8 that I wanted to change to 6/4 for reasons of importing a tempo map from my DAW with quarter note tempo markings. I tried using Select All → Double Note Duration but the end result was a mess.
Essentially the idea would be a dialog box that would allow you to rhythmically scale a passage of music by a specified ratio. For a simple example like above, you would change from eighth → quarter.
Another case I encountered which I had to very tediously edit manually was going from 4/4 to 12/8, e.g. triplet in 4/4 = three eighths in 12/8 and two eighths = duplet. For that example you would enter quarter → dotted quarter.
I’m sure this is likely quite difficult to implement in practice, as it should also adjust time signatures and tempos accordingly if they are selected. But just thought I’d throw it out there and hope it gets tacked onto the end of the long list of things you have planned!