I don't know if it helps you at all (and I'm not attempting to make a "priority" argument of any kind), but the passage in Mark introduces the implied connection of Elijah to John to argue regarding the suffering and resurrection of the Son of Man, according to a theme in Mark in which this is strange to the disciples.
9 As they were coming down the mountain, Jesus gave them orders not to tell anyone what they had seen until the Son of Man had risen from the dead. 10 They kept the matter to themselves, discussing what “rising from the dead” meant.
11 And they asked him, “Why do the teachers of the law say that Elijah must come first?”
12 Jesus replied, “To be sure, Elijah does come first, and restores all things. Why then is it written that the Son of Man must suffer much and be rejected? 13 But I tell you, Elijah has come, and they have done to him everything they wished, just as it is written about him.”
The death of John the Baptist (as "Elijah"), i.e. that "they have done to him everything they wished," is presented in a way that comments on the suffering and rejection of the Son of Man.
The Gospel of Mark doesn't have any statement about John to parallel "the Spirit descending on him like a dove" (Mark 1:10) regarding Jesus, but that statement regarding Jesus may illuminate the way that the text understands John. If the figure of John were similar to the figure of Jesus, then John the Baptist wouldn't
necessarily be the incarnation of Elijah, which might mean that the figure of Elijah at the transfiguration is meaningfully different than the person of John the Baptist, who was buried (Mark 6:29). It's
possible that, for Mark, the relationship of the human John to being in some sense Elijah is similar to the relationship of the human Jesus to being the Son of God, as a matter of adoption and of the descent of a spirit upon him.
As for this part of the text:
2 After six days Jesus took Peter, James and John with him and led them up a high mountain, where they were all alone. There he was transfigured before them. 3 His clothes became dazzling white, whiter than anyone in the world could bleach them. 4 And there appeared before them Elijah and Moses, who were talking with Jesus.
5 Peter said to Jesus, “Rabbi, it is good for us to be here. Let us put up three shelters—one for you, one for Moses and one for Elijah.” 6 (He did not know what to say, they were so frightened.)
7 Then a cloud appeared and covered them, and a voice came from the cloud: “This is my Son, whom I love. Listen to him!”
8 Suddenly, when they looked around, they no longer saw anyone with them except Jesus.
It fits the theme of the disciples' misunderstanding of Jesus, implicitly rejecting the idea that he is simply another in the line of Moses and Elijah. Misunderstanding is behind what Peter says about putting up three shelters. At the moment when the divine voice proclaims him Son, the other two disappear, highlighting the singularity of the role of Jesus as the Son of God.
