Good Question, first of all lets review some verses :
5 And he who was seated on the throne said, “Behold, I am making all things new.” Also he said, “Write this down, for these words are trustworthy and true.” 6 And he said to me, “It is done! I am the Alpha and the Omega, the beginning and the end. To the thirsty I will give from the spring of the water of life without payment. 7 The one who conquers will have this heritage, and I will be his God and he will be my son. 8 But as for the cowardly, the faithless, the detestable, as for murderers, the sexually immoral, sorcerers, idolaters, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death.”
so Here he who was seated on the throne (Jesus Christ) said that
9 Then one of the seven angels who had the seven bowls filled with the seven last plagues came to me and talked with me, saying, “Come, I will show you the bride, the Lamb’s wife.” 10 And he carried me away in the Spirit to a great and high mountain, and showed me the great city, the holy Jerusalem, descending out of heaven from God
so Here an angel of God
8 I, John, am the one who heard and saw these things. And when I had heard and seen them, I fell down to worship at the feet of the angel who had been showing them to me. 9 But he said to me, “Don’t do that! I am a fellow servant with you and with your fellow prophets and with all who keep the words of this scroll. Worship God!”)
here is the angel teling him he is not God , he is a messenger from God
10 And he said to me, “Do not seal up the words of the prophecy of this book, for the time is near. 11 Let the one who does wrong, still do wrong; and the one who is filthy, still be filthy; and let the one who is righteous, still practice righteousness; and the one who is holy, still keep himself holy.”
12 “Behold, I am coming quickly, and My reward is with Me, to render to every man according to what he has done. 13 I am the Alpha and the Omega, the first and the last, the beginning and the end.”
14 Blessed are those who wash their robes, so that they may have the right to the tree of life, and may enter by the gates into the city. 15 Outside are the dogs and the sorcerers and the immoral persons and the murderers and the idolaters, and everyone who loves and practices lying.
16 “I, Jesus, have sent My angel to testify to you these things for the churches. I am the root and the descendant of David, the bright morning star.”
17 The Spirit and the bride say, “Come.” And let the one who hears say, “Come.” And let the one who is thirsty come; let the one who wishes take the water of life without cost.
18 I testify to everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues which are written in this book; 19 and if anyone takes away from the words of the book of this prophecy, God will take away his part from the tree of life and from the holy city, which are written in this book.
- Most Probably He who was seated on the throne (Jesus Christ) said that
He said that after the angel told John not to worship him (the angel), So he who was seated on the throne said that after the speaking of the angel and after showing him all this ..
he who was seated on the throne reveals him self in (revelation 21:16 ) He is God who sent the angel
"I am the Alpha and the Omega," says the Lord God, "who is, and who was, and who is to come, the Almighty."
The revelation from Jesus Christ, which God gave him to show his servants what must soon take place. He made it known by sending his angel to his servant John
Those two verses are Helpful in this point
- also another Probability is that Jesus spoke in the speakings or in the tongue ( if we can say so ) of the angel, Because He is God , He is found everywhere , he can do anything
"Look, I am coming soon! Blessed is the one who keeps the words of the prophecy written in this scroll."
He who testifies to these things says, "Yes, I am coming soon." Amen. Come, Lord Jesus.
- so every one who Testifies To These things would say "Yes, I am coming soon."
So it may be the the language of the heavenly creatures ...
- Last Important thing
God’s name is almost always translated LORD (all caps) in the English Bible. But the Hebrew would be pronounced something like "Yahweh," and is built on the word for "I am."
So (I am ) always means "yahweh " ---> GOD
That's why when Jesus said to the soldiers who wanted to arrest him ( I am )
they kneeled to worship him
so " I am coming " said by the angel can mean " yahwah is coming "