Q: What are your thoughts on adding prayers after communion such as the Prayer to St. Michael the Archangel that many bishops are promoting?
A: Just say no.
The group working on the revisions of the Order of Mass in the years after Vatican II made studied choices about what to remove, what to retain, and what to alter. These decisions cohere extremely well in the missal that we received from the Vatican, and that has twice gone through an upgrade with its second and third editions. The mass that we have does not need any fixing.
Any individual who wishes to pray to St. Michael may do so privately, but there is no space for it as public prayer within the Order of Mass, and it disrupts the flow and meaning of the final parts of liturgy.
If there is a particular concern that drives the addition of a prayer, then that concern belongs in the Universal Prayer. That is why that part of the mass was restored after Vatican II.