I've said before that I like the idea of "Ancient" being a mantle rather than just an honorific. A mantle passed down in a bloodline that contains knowledge and power, and the person to replace Mai would be the new "Ancient". And that the knowledge and power itself helps keep the bearer alive longer, so there are few still alive when Mai took on the Ancient mantle.
I also like the idea of their being five or seven titles and mantles of Merlin, although the bearer doesn't actually have to be on the Senior Council. He could have been Merlin, Gatekeeper, Blsckstaff, and Warden. Maybe there's only one more, or maybe there's three more. Five to match his magical system, or seven for the seven seats.