Keturah
As Sarah stands for "the mother of us all," that is, of those who, by grace, are one with the true Son of promise, of whom Isaac was the type (John 3:6-8); (Galatians 4:26); (Galatians 4:28); (Galatians 4:29); (Hebrews 2:11-13) and joint heirs of His wealth; (Hebrews 1:2); (Romans 8:16); (Romans 8:17); so Keturah (wedded after the full blessing of Isaac) and her children by Abraham may well stand for the fertility of Israel the natural seed, Jehovah's wife (Hosea 2:1-23) after the future national restoration under the Palestinian covenant.
( See Scofield) - (Deuteronomy 30:3).