(in answer to this question on Stack Exchange)
Oh my goodness what a lovely road you have in front of you!
First of all, there’s no right way to construct your future language, just ways that are more or less similar to what we see in real life.
You could argue, for example, that people in the future will stop saying “By God!” in the same way we no longer say “By Thor!” or “By Zeus!” Or you could argue just as convincingly that people in the future will continue to say “By God” (or “Bagad!” or “Pakaa!” or however their phonology goes) as a fossil phrase whose original meaning has been forgotten. The same thing has happened, after all, with such English expressions as “Jovial” (Zeus > Zeus pater > Jupiter > Jove > Jovial) and Thursday (Thor’s Day).
Next, to construct your language, you’ll need to consider the political history and culture of the people who speak it. Have they been influenced by anybody else? Who runs their society? What sort of things do they often talk about? Do they have writing? Printing presses? Radios and television? All of those things will affect your language.
Finally, it’s good to know some rules for how languages tend to evolve. Basically, it’s a balance between the destructive forces of slurring sounds and smooshing words together (*aiwô galīkaz (meaning “every always”)> ǣġhwylċ > ǣlċ > each) and creative forces hyper-correcting, over-enunciating, and adding in words for clarification (like the phrase “each and every.” Perhaps someday we’ll be saying “chnevry” then “chnevery and all” then “nevral” and so on and so on). The specifics, though, are complicated.
And do I have a reading list for you!
As you can see above, it helps to know where English has been when you think about where English is going. I recommend [The History of English podcast.]
For a basic look at how languages evolve:
[The Unfolding of Language]
For how creoles develop (with lots of good examples of English-based creoles)
For examples of other peoples’ attempts to “pre-construct” future Englishes
Justin B Rye’s magisterial [Futurese]
Nick Farmer’s [Belter Creole] (invented for The Expanse TV series)
David Peterson’s [Trigedasleng] (invented for The 100 TV series, a post-apocalyptic scenario similar to yours)
And my submission, [a review of American English from 1800 to 2100]