Are you applying for primary? If so, then I would say it would make sense that the book should be relevant to the pupils you are hoping to teach (ditto if you are applying for secondary English). Aside from that, they want to see what ideas YOU have, not what ideas you are able to garner from online forums. Also, they won't expect to see a finished article; they will simply be looking for someone with the potential grow into a teacher. Come up with your own ideas and you will "own" them, and this will then be apparent in your presentations. Hope that helps.