Normally, there's an established number of board members for each company. On the boards of directors where I'm an active member , if a director leaves is position, it's up to the rest of the board to decide if they replace that director (with the candidate of their choice) immediately or if they wait until next AGM.

If the number of directors is maxed out and they want a specific candidate to be included, the Corporate Governance and nominating Committee Charter will make recommendation to the board to replace a member by a person they think will add value. Of course, that's the formal way it should be done, but it's a lot more informal usually.

So, no need to worry if the "next CEO" is not on the ballot for the upcoming AGM, they'll make room for him.