Advent, as the start of a new liturgical year, is a great place to start using antiphons at Mass. I always recommend introducing them seasonally, since people know it only lasts 4 weeks and are more willing to try something different. At www.antiphonrenewal.com there are many options for entrance antiphon hymns for Advent.
For example, a user recently asked if I could set the antiphons as verse 2 of O Come, O Come, Emmanuel. I was happy to oblige. See the 5 page document here. If any of you need special treatment for antiphons like this, or a new tune, don't hesitate to ask.
As the liturgical reformers wrote: ""What must be sung is the Mass, its Ordinary and Proper, not “something”, no matter how consistent, that is imposed on the Mass. Because the liturgical service is one, it has only one countenance, one motif, one voice, the voice of the Church. To continue to replace the texts of the Mass ... is to cheat the people...texts must be those of the Mass, not others, and singing means singing the Mass not just singing during Mass."