You don't see how this is too broad? let me explain.

When you are creating a flag design, you need to consider the appropriate symbols, geography and colours for the place you are designing for. A flag needs to suit its intended purpose.

In this prompt, I have no idea what place I am designing for. It could be from TV, Movies, Games, Books, Real Life etc. You have not specified the purpose of the flag. Instead you have said the flag should exemplify a trope. But it is up to me to determine what planet/nation/state/city/movement to design for. So long as it expresses a standard trope.

So to do well in this contest, I need to consider all TV, Movie, Games, Books, Real life - planets, nations, states, cities, movements - colours, symbols, geography etc and come up with the combination that would look like a nice flag design and fit a list of tropes.

This is backwards. You don't start with considering everything, coming up with a good design and fit it to a purpose. You start with a specific purpose and create a good design to fit this purpose.

It is not that I don't understand the prompt. It is not that I cannot create a design to fit a standard trope. It is that the contest (if one intends to do well and try and win) is more about picking the best topic, rather than designing a flag for a specific topic.

If I chose a difficult topic, but someone I am competing with (I am looking at you, /u/Imperito) chooses a better topic with richer imagery, they will have a better chance to design a good flag. So it becomes a challenge to choose the best topic, not a contest to design the best flag for a specific topic. Choosing a topic means thinking through all possible options, as specified above, which is way too broad.