It's true that specific details of Melchizedek's life are scarce; The only details we are given of him are found in three verses in Genesis. Melchizedek - his name means "king of righteousness" - was king of Salem (that is "king of peace"). He was high priest of God; Thus he was King and Priest - an important note. He was worthy enough to be tithed to by Abraham (Abram at the time) who gave him a tenth of everything he owned.
That's about all we know from genesis. David affirms the eternal priesthood of Melchizedek in the messianic psalm 110 - The Lord has sworn and will not change his mind - "you are a priest forever in the order of Melchizedek". In so doing, David is affirming the eternal priesthood of the Messiah.
After Christ came, was crucified, and was resurrected, the author of Hebrews is able to identify many more correllations between Christ and Melchizedek in confirming the validity of the Messiah-ship of Christ:
Melchizedek means "King of Righteousness"; He was King of Salem - that is "peace";
The messiah is identified in Isaiah 9 as Prince of Peace and as establishing his kingdom with righteousness and justice.
Like Melchizedek, Jesus united the offices of kind and priest. Previously this was not supposed to happen as God had distinctly established the two offices as being separate - the kingly line coming from Judah and the Priestly line coming from Levi.
This is important because one seeming mark against Jesus Priesthood is that he didn't come from the line of Levi; However, the author of Hebrews points out that Jesus comes from another priestly line - a better one - the order of Melchizedek. He is of the order of Melchizedek, not by lineage, but by "order" - that is, like Melchizedek, Jesus is an eternal priest, without end. Why is the priesthood of the order of Melchizedek a better one than that of Levi? First, because the priesthood of Melchizedek preceded that of Levi. Second, because even Abram recognized the value of Melchizedek's priesthood.
Hebrews 7:4 - "just think how great he was (in reference to Melchizedek). Even the patriarch Abraham gave him a tenth of the plunder! Now the law requires the descendants of Levi who become priests to collect a tenth from the people - that is, their brothers - even though their brothers are descended from Abraham. This man, however, did not trace his descent from Levi, yet he collected a tenth from Abraham and blessed him who had the promises. And without doubt, the lesser person is blessed by the greater. In the one case, the tenth is collected by men who die; but in the other case by him, who is declared to be living. One might even say that Levi, who collects the tenth, paid the tenth through Abraham, because when Melchizedek met Abraham, Levi was still in the body of his anscestor.
And so the superiority of Melchizedek's priesthood is established in order to validate the priesthood of Christ..
So to answer your question, could Melchizedek have been another Christ? He was indeed, in a sense. He is a type of Christ - that is an image of what would come. I think we need to recognize, though, that the presentation of Melchizedek in Genesis is meant to validate, in advance, the validity of the Messiah as priest and king - not the other way around; There aren't enough details, nor does the point of the story seem to be to validate Melchizedek's purpose - other than to foreshadow Christ; Some Christians consider Melchizedek to be a Christophony (a pre-incarnation appearance of Christ in the OT). It may well be true, it may not..Is it important? It seems that we are told what we are supposed to know about Melchizedek in order to establish an "order" that precedes and surpasses the one that follows with Levi...
Was Melchizedek another Christ? He may have been Christ, I don't know. I do know that Christ is another Melchizedek...High Priest forever...