One of the assignments in the seminary's MA Biblical Interpretation course is to identify what kind of midrash Hebrews 7 is. The three possibilities are:
1. prophetic--where an OT text is taken as a prophecy of some current event
2. paraphrase--where an author expands on the meaning of an OT text
3. parable--where an author takes an OT text symbolically or allegorically
Interestingly, many students take Hebrews 7 as prophetic, with Melchizedek as a prophecy about Christ. This usually is expressed in pre-modern terms where the texts are read in terms of the way it looks to me rather than from listening to what the texts themselves say (thus someone might see Melchizedek as a Christophany, something unsupported by the texts themselves but nevertheless a traditional Christian interpretation). I do think there is an element of prophetic midrash in Hebrews 7, but it is more subtle than this.
Certainly Genesis 14 is not prophetic on its own terms. Nothing in this text speaks of any future fulfillment. Hebrews 7 also does not explicitly speak of Genesis 14 as a prophecy fulfilled in Christ, even though it does relate Genesis 14 to a recent event.
The subtle element of prophetic midrash comes in the use of Psalm 110:4, which the author takes as implying that the messiah will be a priest after the order of Melchizedek. It thus implies for the author that the arrival of a Melchizedekian priest brings a change of priesthood. In that sense he does seem to take Psalm 110 as prophetic and thus probably implicitly considers Genesis 14 to be prophetic.
But the primary form of midrash in Hebrews 7 is paraphrase midrash. It expands on what a priest after the order of Melchizedek might be by exploring features of Genesis 14. It uses interpretive techniques like "it doesn't exist if it is not in the Torah."
It also has elements of parable midrash. The interpretation of Melchizedek's name and the name of the town of Salem are clearly allegorical, for example.