This isn't something we're likely to do within the current system. It creates too much opportunity for closed cliques to secure their monopoly on power in particular realms.
Imagine you have King Kepler of Keplerstan, and he names Delvin as his successor. Then Kepler gets himself killed one way or another, and Delvin automatically becomes King.
The next day, who shows up in Keplerstan but Kepler II, son of the former king, and shortly thereafter King Delvin names him as his successor.
This is a simplified example, of course, but it is far, far too easy to abuse such a feature to prevent real change in a realm even if a majority want it.
