Another important question is whether you want to focus on one religious tradition, or whether you want to create a Storybook where you bring together stories from different religious traditions in order to compare and contrast them. Either approach can work wonderfully, so it's a question of what you think would be the best approach for you!
To start your research, you might want to take a look at some of the Bible-based Storybooks that students have done in the past, and there are also some past Storybooks about the Christian saints. Meanwhile, the Storybooks for the Indian Epics class provide some great examples of Hindu projects. You might also get some ideas from the creation Storybooks, too!
Online resources. The Bible is full of stories you could work with, and you can also find many other stories from the Jewish and Christian traditions in the books listed on this Religious Books page; for Islamic story collections, see the Middle Eastern Books page. There's a separate page at this website about Buddhist Jataka stories, and you can find abundant Hindu resources on the India Books page. In general, the Sacred Texts Archive site is full of fascinating materials that you could use for any religion-based Storybook project.
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The widget below shows some Bible stories and Bible characters at random which might give you some ideas if you are interested in a Bible-based topic; you should see a new character or topic at random each time the page reloads: