"I saw in heaven another great and marvelous sign: seven angels with the seven last plagues -- last because with them God's wrath is completed." (15:1). When viewed as a transition to a new era, the book of Revelation begins with letters to churches, then proceeds in steps (seals, trumpets, harvest) toward the completion of God's work on earth.*
"Sea of glass" is also mentioned in 4:6.** "...the sea was to be used by the priests for washing." (2 chronicles 4:6). Fire usually symbolizes refining. So those "victorious over the beast" were forgiven by God and perfected in God's image.
The new song seems to be related to the song of Moses in Exodus chapter 15. The deliverance out of Egypt is similar to, and maybe a model for, the deliverance of God's people out of the world. The new song is likely to be a song of praise to God for delivering God's people out of the world. Colossians 3:15-17 seems relevant here. Also see Revelation 14:3 and 5:8-14.
Is the temple in heaven (15:5) the new Jerusalem? Regardless, no one could enter until the 7 plagues were completed (15:8).
*14:17-20 reads as if it describes the total destruction of disobedience on the earth. If so, then chapters 15 though 18 are an epilogue giving more detail of the destruction. And chapters 19 through 22 are an epilogue giving more detail about the life and surroundings of God's People.
**"Sea of glass" may also represent the calmness of God's rule.
Keep in mind that things often do not happen as one may expect. For example, some people who analyze stock and commodity prices will say that people react in a somewhat predictable manner to situations, but the reaction:
May actually go in a direction that is opposite to what a person would expect -- often to the point where a person thinks he/she has misinterpreted the situation.
May be over ridden by other situations, which captivate people's attention and may have more influence -- at least for a short time.
May be slower than a person may expect.
May not go as far, or as long, as a person expects.
May go faster than a person expects.
May go much further than a person expects -- overshoots -- bubble/burst -- panic!
May last longer than a person expects -- especially the secondary reactions.
All of the above may happen in regard to any specific situation (or set of situations) depending upon one's time perspective: seconds, minutes, hours, days, weeks, etc. Think about this during the seals. "The world is coming to an end" type of panic is likely to happen, especially during the sixth seal. Then, if the half-hour in the seventh seal is actually about 21 years, apathy and misinterpretation are likely to become rampant -- because things are taking too long to unfold! Similar, but perhaps faster, swings in mood are likely to happen during the period of the trumpets -- and possibly even during the harvest.