What would a syntax modifying system look like?
Thought experiment. Imagine you have the ability to modify a language like Javascript or Python, and you had full access to how the language works and behaves and can modify anything.
What would the system/syntax look like that makes the modifications/add ons? What would it have? What would you build/make/add on with it? Include some examples of what you think would fit best and how you would add it with the modifier system.
Side note: I'm not asking for if this is a good idea for a language to have. I'm just asking what would it look/feel like if it was a thing.