Firstly, I'd point out that as this is a slightly open-ended question I'm not certain how well it fits in with the guideline. I'm hoping that the fact there's a way to define an answer means that it is not necessarily wrong.
So, the question came to me after reading this article here: English is not normal. What stood out for me was how the author described that a method of judging the formality of what is being said is through the origin of the word used -- the author also brought two distinct examples ('help'/'aid'/'assist' and 'kingly'/'royal'/'regal') where the Old English > English word is the least formal of the possible options, the French word is the middle ground and the Latin derivation is the most formal.
What other pairs of this kind exist, and is this the best way of determining formality? Does this mean that the best way to make a statement less/more formal is replacing words with their synonyms based on the origin of the synonym (example: "Kingly help was given"/"Royal aid was presented"/"Regal assistance was provided")?