Who would obtain Coordinate's ability if Historia was killed by the MP?

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.