Revelation 15:1 forms an inclusio with Revelation 12:1, 3: Both speak of a “great sign in heaven.” There is progression from chapter 12 to 15: At the beginning of the sequence, the dragon and woman are in heaven. In the course of chapter 12, both leave. When 15:1 returns to a great sky sign, it’s not the dragon but seven angels bearing the seven last plagues.
More locally, 15:2-4 is clearly a completion of a textual arc that begins in 14:1-5. The 144,000 on Zion hear music from heaven, and learn to sing the new song. By chapter 15, the camera has moved from Zion to a position above the firmament, where the harvested saints sing the song of Moses and the Lamb.
So, 15:1-4 completes a section of text. At the same time, 15:1 also inaugurates the section that runs through chapter 16. The seven angels with plagues are given bowls of wrath (15:7) and then they are instructed to pour them out into the land (16:1).
The transitional chapter 15 has an internal structure of its own, marked by a careful incluio. Following the Greek word order, we have this:
A. Angels seven
B. having plagues seven, the last
C. because in them is finished
D. the passion of God (v. 1).
E. Victors sing song of Moses and the Lamb (vv. 2-4).
X. I saw: Temple opened, seven angels with seven plagues; seven angels receive bowls (vv. 5-7a).
E’. Angels wear clean, bright linen with golden sashes.
D’. full of the passion of God (v. 7b).
C’. no one can enter the temple until finished
B’. the seven plagues
A’. of the seven angels (v. 8b).
Not only does the angel-plague combination turn around (from angel with plagues to plagues of angels), but the placement of the number changes. In verse 1, the number comes after the noun, while in verse 8 it comes before the noun. (Verse 6 has the number before the noun).
So the textual chunks 12-15 and 14-15 hook into chapter 15 at the beginning; but the beginning of chapter 15 also throws a line ahead to chapter 16; and in the middle of these overlapping structures is a passage with a neat internal of its own.