After these things
This phrase is used to denote a distinct vision. While it may be related to the previous vision, this vision will have its own characteristics. That is, in chapter 17, we saw the collapse of the pseudoreligious empire (of Babylon). In this chapter, we will see the conclusion of the destruction of Babylon, but in particular, we will see the political and economic empire(s) collapse. Note how the vision changes the other times in Revelation that "After these things" is used (Revelation 4:1, 7:9, 15:5, and 19:1).
I saw another ANGEL coming down from HEAVEN,
This is another ANGEL like the seven angels who carried the seven plagues (Revelation 15:5-6 and Revelation 17:1). This ANGEL, too, was a 'presence angel' coming from the temple in heaven.
Some scholars suggest this may be Christ himself and use John 8:12-20 to support their argument. Given the precision of Revelation throughout, it is not likely that Christ would be denoted as an ANGEL here. There is no argument that Christ has GREAT (GREATEST, actually) AUTHORITY and could easily fill the earth with His GLORY. But here this part of the vision is given to an ANGEL.
Scripture says little about the AUTHORITY given to angels. Probably because of the "bad experience" with Lucifer, scripture downplays this side of angeldom. We know that some powerful angels are given control over physical phenomena (see discussion in Revelation 16:5). This angel compliments these other strong angels as the "angel of GLORY(light)".
Those who believe this may be Christ find support in John 5:25-27-29 and Acts 10:34-42-43.
This ANGEL must have come from the presence of God and His unapproachable light (1 Timothy 6:11-16). While this is not a common phenomena, Moses glowed with God's GLORY after having spent time in God's presence (Exodus 34:29-35). We could argue that we don't see this GLORY today because we spend so little time in God's presence (or maybe we are selfish and don't want to). Finally, after a world of darkness (the fifth bowl - Revelation 16:10), the brilliance of this GLORY must be shocking. Most importantly from a spiritual viewpoint, this is a DIRECT CONFRONTATION of one ANGEL of GLORY(Light) against the 'disguised angel of light' (2 Corinthians 11:7-14-15) who is behind this Babylonian empire(s).