Hi, this Wiki explains how to use Stageview and provides a good starting point:
https://manual.openlp.org/stage_view.html
You don't need a plugin to customize the Stageview. However, if you want specific features to control OpenLP or retrieve data from it that aren't available through the API, then you'll need to create a plugin written in Python to suit your use case.
At our church, we actually do this — but it's completely custom-made to fit our needs. We also have to verify everything works properly after each update, so it's not exactly user-friendly unless you're willing to put in that extra effort.