Prophetic Fulfillments in Mark

Your question seems to assume a dichotomy between "fulfillment" of the Old Testament in Mark's Gospel and bringing "something entirely new" to the table in regards to Jesus. But it is not really a case of "either à or" but of "both à and." Since time and space will not permit me to address every instance of this in Mark's gospel, I'll simply provide some examples.

For instance, in Mark 1:2-3, Mark refers to the book of Isaiah:

I will send my messenger ahead of you,
who will prepare your way -
a voice of one calling in the desert,
Prepare the way for the Lord,
make straight paths for him.

Mark 1:4 indicates that this "messenger" is none other than John the Baptist. But things get interesting when we actually turn to Isaiah 40:3-5, the original source of Mark's quotation. In this latter portion of his book, Isaiah was writing a message to the future exiles in Babylon, assuring them of God's power to restore the Israelite nation (40:1-2). He referred to "a voice of one calling in the desert" (probably an angelic messenger) commanding the construction of a "superhighway" that was to run through the desert between Babylon and Jerusalem:

Prepare the way for the LORD;
make straight in the wilderness
a highway for our God.
Every valley shall be raised up,
every mountain and hill made low;
the rough ground shall become level,
the rugged places a plain.
And the glory of the LORD will be revealed,
and all mankind together will see it.
For the mouth of the LORD has spoken
(Isaiah 40:3-5).

Isaiah's original audience would have recognized that the purpose for constructing this metaphorical "superhighway" was to prepare for the coming victory processional for the parade of triumphant Israelites out of the captivity of Babylon back to their home in Jerusalem. Its real-life counterpart was probably the Way of Marduk, a Babylonian ceremonial highway built to celebrate Marduk for giving the Babylonians victory over their enemies.

God was offering his people a miraculous, glorious restoration after "her hard service" had been completed in Babylon. But as you may know, continued faithlessness on the part of Israel meant that the actual restoration to Jerusalem was to be only a dim reflection of what had been offered. With respect to the gospel of Mark, one thing is certain: none of Isaiah's original audience would have understood this passage to be a prediction of a locust-and-honey-eating prophet in the desert many centuries in the future.

Knowing all this, then, it becomes obvious that "fulfillment" as it is understood in the New Testament does not usually mean a literal realization of a specific prediction from the Old Testament. Instead, Mark and the other writers are making use of a literary form known as typology.

As another example of this, in Mark 7:6-7 Christ quotes from Isaiah 29:13 to rebuke the Pharisees, saying that Isaiah had "prophesied" about them. But of course, Isaiah was writing about individuals who had perished many centuries before. Jesus was indicating a typological fulfillment on the part of the Pharisees, and they knew this, so they did not correct him by pointing out that Isaiah was talking about somebody else. They knew full well what Jesus was saying about them. Jesus himself experienced numerous typological fulfillments.

These typological fulfillments are especially easy to see in the Gospel of Matthew, but they crop up in Mark as well. For instance, in Mark 12:10-11 we see Jesus claiming Psalm 118:22-23 as being fulfilled in himself, whereas the psalmist originally had the king of Israel in his own day in mind. It can rightly be said that Mark and the other Gospel writers were showing that Jesus "fulfilled" Old Testament prophesies, while at the same time they were offering up "new" revelations of the reality to which the Old Testament types pointed.

Beyond such fulfillments, Mark added much new material beyond that found in the Old Testament. That is to say, he included many details in his gospel that were not specifically or even generally prophesied in the Old Testament. Perhaps the greatest such detail is that the promised messiah turned out to be God incarnate — a twist that was not expected in the Old Testament.