Hello.
I thought the following idea that could make more interesting the cleric class and also more dynamic.
Actually clerics unlike D&D don't choose their domains, they get two from all that his deity offers.
For example: Clerics of Mystra have as domains Magic Domain and Illusion Domain, but the possible domains for a cleric of Mystra in D&D are: Good, Illusion, Knowledge, Magic, Rune, Spell.
Of all the domains that a cleric of Mystra could have access only the two for default and Knowledge are coded, what I suggest is creating quests that upon completing you lose one of the two domains (your choose) and you get the one that the quest was aimed for, for example a quest of Mystra involving the Knowledge domain must have in consideration Mystra's dogma and the thematic of the domain that you want to acquire,
like making a long quest that involves the study of dead magic zones.
If more domains are coded, it could be possible create quests involving that domains eventually, though the idea is that you can change a default domain for a new one, no changing domains at will whenever you want, so if you change your two default domains you couldn't change your domains ever.