I have heard the Transfiguration theory by various conservative interpreters, but it is clearly wrong. The prediction in Matthew 16:28 (= Mark 9:1, Luke 9:27) is part of the Son of Man eschatology influenced by 1 Enoch 1:9 which expects a future epiphany of God or his divine agent accompanied by his angels to execute judgment on the earth, giving everlasting glory to the righteous and everlasting punishment to the wicked. See Jude 14-15 which quotes 1 Enoch 1:9 ("Behold, he comes with the myriads of his holy ones to execute judgment on all"), 1 Thessalonians 3:13 ("May he strengthen your hearts so that you will be blameless and holy (e.g. being judged holy) in the presence of our God and Father when our Lord Jesus comes with all his holy ones"), Matthew 25:31-46 ("When the Son of Man comes in his glory, and all the angels with him, then he will sit on his glorious throne. Before him will be gathered all the nations, and he will separate them" in judgment, giving some of them the kingdom and others eternal punishment).
That Matthew 16:28 belongs to this apocalyptic tradition is indicated by the preceding verse:
"For the Son of Man is to come with his angels in the glory of his Father, and then he will repay every man for what he has done. Truly, I say to you, there are some standing here who will not taste death before they see the Son of Man coming in his kingdom" (Matthew 16:27-28).
The reference here is to the judgment of "every man" occurring when the Son of Man comes, and it is this event that will happen before those standing in Jesus' presence would taste death. This is the same event discussed in Matthew 25:31-46, which in the narrative of the gospel is uttered in the Olivet discourse after the Transfiguration, and which refers to a still future judgment of the world. At Jesus' trial, he similarly tells those present: "In the future you will see the Son of Man sitting at the right hand of the Mighty One and coming on the clouds of heaven" (26:64). This is essentially the same prediction in 16:28, that Jesus is telling those judging him that they will see the Son of Man coming on the clouds of heaven (e.g. to execute judgment on THEM). The Olivet discourse itself has another prediction similar to 16:28 which also follows a description of the coming of the Son of Man in judgment:
"Immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light, and the stars will fall from heaven, and the powers of the heavens will be shaken, then will appear the sign of the Son of Man in heaven, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory; and he will send out all his angels with a loud trumpet call, and they will gather the elect from the four winds, from one end of heaven to another....Truly I say to you, this generation will not pass away till all these things take place" (Matthew 24:29-34).
In v. 34 is a prediction just like that of 16:28, and again it refers not to a past event (e.g. the Transfiguration) but to the future coming of the Son of Man in judgment and glory. The language used is the same: the Son of Man, coming in glory, accompanied with the angels. Compare also with Revelation 1:7: "Behold, he is coming with the clouds, and every eye will see him, every one who pierced him, and all tribes of the earth will wail on account of him". Note that in Matthew 24:34 the reference is to "ALL THESE THINGS" as having occurred, which include not just the destruction of Jerusalem but also the coming of the Son of Man on the clouds of heaven to gather the elect into his presence.
The Transfiguration theory ignores the crucial fact, that the coming of the Son of Man in glory is a reference to the final eschatological judgment of the world.
Moreover, there are other passages in Matthew that imply that this coming of the Son of Man will occur within the generation of those who saw Jesus. Cf. Matthew 10:23: "When you are persecuted in one place, flee to another. I tell you the truth, you will not finish going through the cities of Israel before the Son of Man comes".