When you’re writing a long series, it’s common to have ideas that aren’t full formed or won’t be needed until future books. You can slot all these ideas into this section so you don’t find yourself forgetting what’s been written and what hasn’t. It’s likely some ideas will be abandoned for whatever reason, but you don’t want to forget which ones.
You can organize this section however you like. If you’d prefer to keep everything on one page, that’s fine. If you’d prefer a new page for each idea, you can do that too.