I think you are confusing Old Testament prophecy which has passed into history at the death of John the Baptizer, and New Testament prophecy which is not intended for the Gentile nations, but for the church meeting.
As far as "thus says the Lord", I don't go along with that at all, and never say it when I give a prophetic word. This is because if a word is prophetic for the hearer, then I don't have to say it is from the Lord at all. That is for the listener to determine.
Jeremiah 23:25-40 is a good Scripture passage to encourage a person to be extra-careful to make sure that when he goes to give a prophetic word either to a group or another person, he is in tune with the Holy Spirit. To be safe, it is best to give a word by way of advice rather than preface it as from the Lord. The only direct voice of the Lord is in His written Word, and that is what He speaks through. If a person hears a voice that says things not consistent with what God has already said in the written Scripture, then the voice is not coming from the Holy Spirit, but from another spirit.
Even to be able to say, "If Jesus was here, He would say this to you", one must be mighty in the Scriptures and a person of prayer so that he can be totally familiar with what comes from God and what doesn't.
Also, I don't believe that predictive prophecy is the type of prophecy that Paul taught in 1 Corinthians 14, but can be more like New Age fortune telling.