Written languages are not static. You can absolute introduce your own vocabulary and grammar. Yes, you might become harder to understand for someone unfamiliar with it but so can get a Lisp program when macros are over used. It is an art after all.

This is especially insane when talking about English. What English? American English? What group? African-American English that introduces whole new grammatical concepts? Indian English? Even two people grown an raised in London can have wildly different ways of speaking depending on their class backgrounds and many other factors. There isn't one English.

And it is ever evolving. Shakespeare English is vastly different than 21st century English. In fact Shakespeare himself invented roughly 1.7k words that we still use today.