There are three ways to write historical fiction about the future.

Fiction set in the past often reflects the time in which it's written. George R. Stewart's 1950s novel The Years of the City is about the rise and fall of a Greek colony in southern Italy. It's well researched, so far as I can judge; I didn't find any anachronisms. But there's a chapter about McCarthyism in Ancient Greece. And other matters are given emphases different from those a writer of our time would give them.

For historical fiction set in our past, but giving a glimpse of the future, all that's needed is to create an implied author who has the preconceptions and biases of your fictional future. And, of course, to convey this information to the reader.

Historical fiction set in our time, giving a glimpse of the future, probably requires less research. Again, you need an implied author who's writing in the future you've constructed.

If you find these methods too easy, try setting your historical fiction in the future. The implied author would be someone in the farther future.