So, when it comes to writing ideas there's a few different approaches that I think come naturally.
1) Read up on a handful of details about the group you're writing ideas for and create an unfocused but interesting hodge-podge of things. Zoroastrian example: They have interesting funerary rites, Xwedodah marriages and their holy book is written in an ancient dead language that forms their language of liturgy. They should get X bonus for the Towers of Silence, Y bonus for the Xwedodah, and Z bonus for the Avesta. This approach is sometimes called "top-down."
2) Base the ideas solely off of the mechanics of their neighbours with a handful of differences. Zoroastrian example: taking Muslim, Ottoman, Indian, or Persian ideas and picking and choosing bits and pieces from each in order to make an idea set that is different, but "fits" the region.
3) Design the ideas based off of the playstyle you imagine for your specific group. Zoroastrian example: The religion has lasted forever, and will likely be surrounded by nations of other, hostile, religions. It will likely be in Persia, and so will be affected by Persian geography. It will likely only appear on nations of non-Western tech groups. Therefore, the ideas would be related to making your religion harder to convert, or hostile core creation costs increased, or tolerance for other religions (since if you expand you'll have other religions within your administration), Persia is mountainous and coastal, so defensive bonus and sea trade/naval things would help, and some sort of thing that will help them either Westernise or reduce the effect of not Westernising.
Out of the three I'd prefer the third method. Obviously in the end, you're the one writing the ideas, so you'll decide what's best. I hope that this is found to be helpful by at least someone. Hopefully what I wrote can act as a foundation for creating a more critical approach to writing the new content.
Of course, any modifications/additions/criticisms of/to the approaches I've laid out would be helpful as well.