I created a plugin for myself because I keep all my charts in concert key (for guitar, piano, voice, etc.), but have to transpose them any time there are changes for Bb sax and trumpet. This is obviously not that hard to do in MuseScore, just tedious when you are maintaining many dozens of scores. I wanted an automated way of transposing up a step and adding text to indicate the chart is for Bb instruments. This was not so easy to figure out with the new API (for me), but I finally got it to work pretty well, using examples from other plugins. If this was generally useful it could certainly be adapted to accommodate other keys and instruments, but it suits my current purposes as is. Maybe useful example for others to use.