I would recommend something similar to the SHOGI promotion rules. [1A] is your original promotion rule (mandatory for pawns). [1B] 'A promotion may be deferred to a later move, as long as this move begins inside the promotion zone. Where the move ends is of no relevance.'
According to my limited knowledge of SHOGI, a Bishop in the promotion zone will first move like a Bishop and then have the option of promoting to a Dragon Horse. This sort of promotion can make Multi-King Chess more exciting.
According to my limited knowledge of SHOGI, a Bishop in the promotion zone will first move like a Bishop and then have the option of promoting to a Dragon Horse. This sort of promotion can make Multi-King Chess more exciting.