My approach with specialty priests, even in settings I created, is to offer the player the chance of picking one of the options in the books I have, if there's one with powers that fit. If nothing pleases the player, then I'll try my hand in building a homemade specialty priest for that specific campaign.
Saying you need to create each specialty priest of the pantheons in your game to offer as an option to players is like saying that you need to create the prestige class of every organization in your world, so players can make informed decisions while leveling up. Not necessary, or even desired, in my opinion.
I did that too. I preferred the original purpose of prestige classes -- class choices that were a bit more narrow than the base classes tied into the world organisations and lore -- more than what they evolved into.