I would like Three (3) Senior Co-Leader positions. Three max.
Ordinary co-leaders can no longer promote members to co-leaders.
A Senior Co-leader can promote member to Co-leader.
If there is an available Senior Co-Leader place, any Senior Co-Leader can promote a Co-leader to Senior Co-leader.
Any two Senior Co-Leaders can demote the third.
The Leader can demote anyone. The leader can promote anyone, subject to max 3 Senior Co-leaders and max 1 Leader.
The reason for this is that there are too many co-leaders, the position is devalued, and it is too hard to make up a rule to stop co-leaders eventually making everyone co-leader. However, if no one but an absent leader can promote to Co-leader, the one leader able to promote to second level is too few. Three, I think, seems about the right number.
Among the three, two can demote the third. I expect this rare event will usually be used when the leader is inactive and the third co-leader is inactive, and there is a better candidate.