My understanding is that Ephesians 4 teaches that some believers are called to equipping ministries--these include apostles, prophets, evangelists, pastors and teachers. All of these are necessary for a healthy balanced church, but the way in which these equipping ministries are represented in the church today is very different from the way they were exercised in the first century. I won't pursue that topic in this posting, but I do want to make clear that there is and will be a continuing need for ministers who "equip God's people for the work of ministry" in the church fifty years from now.
In the future we will still have those who serve the church full-time as staff ministers. At the same time, we will have many part-time or bivocational ministers serving the church in various roles. The most interesting thing about these "professional" ministers will be where we will find them and how they will be trained.
First, many of these staff ministers will come out of the church in which they are already members. Their gifts will be recognized through the exercise of those gifts in the church, and their calling to ministry will be not only confirmed but often initiated by the congregation.
Second, they will receive their theological education through satellite centers of seminaries (see Central's "teaching church seminary" model www.cbts.org), directed individual study in consulation with veteran ministers, and cohort groups led by qualified ministers with specialized theological degrees. In other words, the seminary will come to the students rather than vice-versa. This will not only allow them to remain in their places of ministry but to enter into a dialogue between praxis and formal education.
Third, there will be more itinerant ministers pursuing their ministries either outside of the church or in conjunction with several churches. Many of these will be "apostolic" ministries, taking the gospel to new areas or new groups. The most healthy arrangement will be for these folks to be commissioned by a local church or group of churches. Such an arrangement will provide flexibility, stability, and accountability.
A final word here about those with advanced theological training--PhDs, etc. We will still need these folks, but they will not be teaching on seminary campuses. They will be ministers in local churches who also teach, teachers who are themselves "circuit riders" (traveling from place to place to teach groups of students), and teachers in colleges and universities who also "do theology" in churches.
Ministry in fifty years will be both interesting and challenging!