I think this would be a good time to actually get a Council going that resembles the one from Star Wars canon.
In both the Old and New Jedi Orders, there were 12 members of the High Council - which resided on Coruscant. There is no need for all seats to be filled or held by player-characters, though. Star Wars canon even has some New Jedi Order High Council seats be vacant for years.
But, I think we could work something out by using the Old Jedi Order's High Council as an example. The membership of the High Council includes:
- 5 Lifetime Members
- 4 Long-Term Members
- 3 Limited-Term Members
All are Masters (usually. see: ROTS)
The five Lifetime Members could easily be filled by player characters at all times. They only leave their seats by stepping down or dying. The Long-Term Members had more flexible terms of service. For example, the Jedi High Council could appoint someone for a Long-Term Service by saying their approved term is for 10 years, while another Long-Term Member was approved for 15 years. We could use real-time if player-characters are appointed to such seats.
The Limited-Term Members have very short terms in comparison to the Long-Term Members. The point of the Limited-Term Members is to have different ideas being presenting to the High Council, especially in times of crisis.
We could then use the rule in the New Jedi Order that appointments to the High Council must be unanimous among its members (this is why we can explain seats are empty). If anything, NPC's can fill the remaining seats - and then they inexplicably die when the player-characters wish to appoint another player-character.
The title of Grand Master is not a title passed down by the predecessor in canon. Instead, it involves the High Council unanimously choosing someone to be the recognized head of the Jedi Order (and supposedly can be more than one person, if the High Council desires).
Titles such as Battle Master are unrelated to the High Council, except that the High Council usually is the body that bestows such titles to people.