2) There are those parts (far fewer) that can be in “these or similar words.” That is, there is a recommended text, but the celebrant is free to personalize it although it is with the understanding that it must convey a similar meaning. So at the beginning of the penitential rite it says, “. . . the priest invites the people to recall their sins and to repent of them in silence. He may use these or similar words.” He certainly cannot talk about how the Cleveland Indians are doing (or not doing) but the moment can be more personalized.
3) Then there are those parts that the priest is to create. Sometimes it is optional such as with the introduction of the Mass. “The priest . . . may very briefly introduce the Mass.” The largest section where this occurs is the homily. Outside of some guidelines it is completely generated by the priest. It is also one of the few parts of the Mass that the congregation is not required to say amen to.
Creativity does not come from throwing off all constraints but finding a way of creating something with restraints. And over rigidity where it is not needed can make even heaven on earth seem a trial. And to be quite honest, its more fun that way too.