What are the most probable changes in grammar and vocabulary of English in the 21th century?

OK, here’s one.

’ve after modals has already been reanalysed to of; not just as a written form, but in spoken English: would of, could of, should of.

Prediction: this gets expanded further by analogy, to link other modals and auxiliaries, now that the of is no longer analysed as a verb. can of, had of.

Stranger things have happened.

No, not Stranger things of happened. There are grammatical constraints at work here.