Bartimaeus begs the King for mercy. This request is quite general, certainly for a beggar who is blind. Jesus will ask for further desires (Mark 10:51). Bartimaeus wishes above all to share in the mercy (not in the wrath) of the coming King.
We do not know that performing miracles belonged to the Jewish expectation with respect to the coming son of David; The prayer, Have mercy on me,
is more a prayer to God than to a human being. But we do not know everything about the Jewish expectations regarding the son of David; in fact, we know but little about this. Moreover, the Jews in Matthew 12:23 conclude, just because of a great miracle of healing, that Jesus could possibly be the son of David. It would also be strange if outside Israel we meet expectations of miracles around certain great rulers and that these would be absent among the people of Israel, while they possessed the illustrations of the coming King, as in Psalm 72:1–20. It is certainly also incorrect to assume that the Jews connected to the son of David
only national-political ideals.
Also the rather national book Psalms of Solomon (first century BC?), where we find the name son of David,
say of him, So that peoples from the ends of the earth come to see his glory, bringing as present their exhausted sons, to see the glory of the Lord wherewith God has glorified Jerusalem. He rules as righteous king over them, taught by God, and in his days no injustice happens among them anymore, because they are all holy and their king is the anointed of the Lord
(Psalm of Solomon 17:31-32). The prayer for mercy,
which can also occur with respect to men, certainly fits a King who will rule as God’s anointed.1
47 And when he heard that it was Jesus of Nazareth, he began to cry out and say, “Jesus, Son of David, have mercy on me!”