Keturah,

The Scriptures nowhere authorize the introduction of Keturah as a figure of a covenant. So far as Keturah herself is concerned, she was never a wife, as Sarah was; she was not even a servant of a wife, as Hagar was the maid of Sarah, and as the Law Covenant has been in some sense a servant of the New Covenant. (Gen. 16: 1-3; Heb. 8: 4, 5 ; 10: 1; 1 Cor. 10: 1-11; Gal. 3 : 24.)

The word translated “wife,” in Gen. 25: 1, is ” ishshah,” which has a very wide range of applications, both good and evil, and is so used in Scripture. (In Gen. 16: 3 is found an illustration of the range of this word, in that both Sarah and Hagar are described as “wife,” though it is most obvious that the best sense of the word could not have applied in Hagar’s case.) The word translated “concubine” in 1 Chron. 1 : 32, is “pilegesh.”
It has no other meaning than that given in the text.
These are the only places where Keturah’s name is mentioned, and they undoubtedly show her position in Abraham‘s household.
The children of Keturah, so far as Scripture informs, we're not godly people. Some of them were very much the reverse of godlike, and were the enemies of God’s people. They were not such as could typify children of God by a covenant, especially not the New Covenant. The Midianites, descendants of Keturah, were particularly the enemies of Israel, and some other tribes descended from her were in doubtful, if not unfavourable, positions. — Num. 25: 17; Jer. 25: 23; Ezek. 27: 15, 20, 22, 23; 38: 13.

It is safe to leave the Apostle’s allegory as he left it, and not suppose that because Abraham’s wife and her bondmaid were used as figures of divine covenants, his concubine Keturah is to be so regarded. If Keturah, why not the others? For Abraham had more than one in that relation, though their names are not known. — Gen. 25: 6.