I think it would be good to have a metrical verse version of the introit for each of the feasts that call for using Gloria in excelsis at mass (because those are the ones I lead music at.) You could provide a second verse and doxology that could be added to the proper verse for each of the main categories of these feasts: Apostles, feasts of our Lord, Marian festivals, evangelists. Having a hymn that names the festival and that can fill out two or three verses would certainly be useful to have in our hymnbook. I would also favour providing several tunes for variety's sake.