From a recent XKCD, via Language Log, comes this concise observation on the future history of the English language. (Don't forget to go to XKCD and do the mouse-over.) Over a somewhat longer time scale - but just how long is very hard to say - the spoken English of the future will become unintelligible, with the written language following at (perhaps) a slightly slower pace. There's at least one online speculation about 'Murrican in the year 3000, though the website is currently down.
Given how widely it is spoken, I can readily imagine future English dividing into an 'Anglic' family of languages, as Latin fissured into the Romance languages. I seem to recall Arthur C. Clarke arguing that none of this would happen, because sound recording would freeze the language. He was wrong; apparently even the BBC no longer uses 'classical' BBC English.
The rate at which this might happen is not known. There have been suggestions that language change, like genetic drift, is random in detail but happens at statistically predictable rates. But the known rates of language change vary widely. English has changed beyond intelligibility in the last thousand years*;
Islandic Icelandic [oops!] has changed only slightly, and I've read that classical Greek is about as accessible to modern Greeks as Chaucer is to us.
* Written English artificially exaggerates this, because Old English had very different spelling rules, but the spoken language would still be unintelligible. The biggest change wasn't all those French words the Normans brought, but the near disappearance of the Germanic grammatical case system - probably already fading from the spoken language before 1066.
In any case we can only speculate about future English; we cannot predict it. For story purposes, however, we might want to evoke it. Inventing new slang words is an ever popular trick, though it is more common (and more interesting) for familiar words to acquire new meanings, as 'text' has now become a verb.
Over longer time periods the rhythm of speech changes, which ultimately brings changes in grammar. 'Yoda I am' is a familiar example, long since beaten to death. Firefly used a mix of invented curse words and some subtle shifts of speech rhythm to effectively convey a different era. I once played around with using a pseudo 18th century diction to represent 'Standard English,' used in the 28th century as medieval people used Latin; a character trying to
come on to communicate with a girl in a foreign station describes a trade starship as an Indiaman.
Over longer time periods recognizable features would disappear, and a future language can only be represented by tone. The challenge then is a familiar one, equivalent to people in a fantasy novel speaking an invented language, or Marcus Didius Falco speaking a slangy, streetwise Latin.
Related Post: My last look at language speculated about communication with aliens.