A red line is
a: a line you should not cross
b: a path a story follows. For example Frodo in lord of the rings, or visually the red lights that represent the red line of germanys history in the song "Deutschland"
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I would not apply a red line on Game mechanics. Game mechanics need some basic rules, but these can change during development. Specially because these basic rules are given by Natural laws and Math. Imagine a city with its streets, if you change a small Street suddenly the huge streets work better or worse, sometimes so worse that you need to change them (Yes i play citys skylines at this moment, so this "Picture" comes to my mind)