Currently, you can only display pages as pages. However, there is a preview button that shows the content in a modal. I think that some of the content would be better as a modal. It would be great if we could specify whether the content is displayed as a page or as a modal. I do get that this depends on the page it is linked from, but would imagine there are ways to deal with this. I've created modals on pages via jQuery with no problem.
Couldn't one just build a modal inside of a custom page?
You could, if you want the modal on top of that page. But if you want the modal to pop up on some pre-existing page, it would need a little more work. Like if I was going to add some information that pops up from the Circulation page or Cataloging page, where I don't want to navigate away, and instead just have that information come up wherever that link was, that would be a little more complex.