When MP killed Historia's mother they also wanted to kill Historia. In that time Reiss's family was dead so what was Reiss/central government plan to obtain Coordinate power if no one is left of the Reiss bloodline?
The MP had no plans. However Rod Reiss, Historia's father, was still alive at the time and could theoretically have taken the coordinate himself or fathered more children to do so. Provided he could consume Grisha (or later Eren). Also, when Karl Fritz and family moved and changed their name to Reiss, some of the family stayed in what became Marleyan territory. They are not mentioned thereafter, but some could have survived.