You err in than you cannot distinguish between Prophets, Prochecy, and the LOGOS or written word of God
Someone who moves in the office of a prophet is NOT adding to the cannon of Scripture as you contend. Indeed we agree that he/she must align whatever prophetic word they claim to have with the Word. Thus, your claim that John was the last prophet is a legalistic and incorrect interpretation of Scripture.
The gift of prophecy by its very nature God speaking to his people TODAY. And God has set up the offices of ministry to the church, one of which is the office of Prophet.