For simplicity we'll say the candidate can't be a current member of the council.

That's not simplicity, that's a major factor.

The closest existing example is the election of the Pope in Rome. The Pope appoints cardinals and when he dies the cardinals choose the new Pope, normally from among their own number. However you've forbidden the council from following this path. If the current emperor wants to remove someone from the possible succession, his easiest path is to appoint them to the council. It's a great honour that they can't refuse, but it takes them out of possible consideration.

The electors of the Holy Roman Emperor mostly elected the eldest son of the last emperor. If you're following that path then it's just a formality, but equally this was a fixed set of persons, the position of elector was hereditary.

For simplicity we'll say the candidate can't be a current member of the council.

That's not simplicity, that's a major factor.

The closest existing example is the election of the Pope in Rome. The Pope appoints cardinals and when he dies the cardinals choose the new Pope, normally from among their own number. However you've forbidden the council from following this path. If the current emperor wants to remove someone from the possible succession, his easiest path is to appoint them to the council. It's a great honour that they can't refuse, but it takes them out of consideration.

The electors of the Holy Roman Emperor mostly elected the eldest son of the last emperor. If you're following that path then it's just a formality, but equally this was a fixed set of persons, the position of elector was hereditary.

For simplicity we'll say the candidate can't be a current member of the council.

That's not simplicity, that's a major factor.

Consider the election of the pope, the cardinals choose the new pope, normally from among their own number, but you've forbidden the council from doing this. If the current emperor wants to remove someone from the possible succession, his easiest path is to appoint them to the council.

The electors of the Holy Roman Emperor mostly elected the eldest son of the last emperor. If you're following that path then it's just a formality, but equally this was a fixed set of persons, the position of elector was hereditary.

For simplicity we'll say the candidate can't be a current member of the council.

That's not simplicity, that's a major factor.

The closest existing example is the election of the Pope in Rome. The Pope appoints cardinals and when he dies the cardinals choose the new Pope, normally from among their own number. However you've forbidden the council from following this path. If the current emperor wants to remove someone from the possible succession, his easiest path is to appoint them to the council. It's a great honour that they can't refuse, but it takes them out of possible consideration.

The electors of the Holy Roman Emperor mostly elected the eldest son of the last emperor. If you're following that path then it's just a formality, but equally this was a fixed set of persons, the position of elector was hereditary.