The best extra text sometimes comes from knowing what issues your core text is targeting, you could by theory select any text as an extra text but if you're looking for some formula or method to having the best text it's good to deconstruct your core text in terms of main ideas and themes.
Firstly, it would be a great idea to watch the documentary 'Go Back' in it's entirety so you can start to brainstorm what aspects of discovery it is targeting. At the same time you'll be exposed to the world issues and main thematic concerns, this will make for a healthy table of notes or simply a solid understanding before school starts.
After that, I would try and recall what texts you might have studied in previous years that could best link to the ideas of Go Back as this is possibly the fastest way of selecting a text that you will know the complete analysis for.
If no text suits your criteria then it's time to broaden your horizon and start the search for other texts, as your doing a visual text as your core it is recommended you look to literary texts like novels, poems, news articles, short stories, etc. From this refined list you can then look at popular classics and read their synopsis or blurbs to see if they address the same issues, once you find one that suits read it! P.S. selecting a popular text (classic ,etc) is sometimes a good idea as there will be online resources for you to cross-reference your understanding to ensure you have a correct interpretation.
Hope this helps!
