Warhammer 40k Seems to Be Doing Well These Days

Is it because it keeps having the same problems it's always had while other genre franchises have been trying to kill themselves as inventively as possible? Yes.

Addendum

All books. Already you should have AIs reading everything for you, and that's only going to be more true as we proceed into science fictional realms.

Authenticity

To give readers a real old-timey experience, books should be writtenwithoutspacesorpunctuationnoidonotmeanjustfantasy

Addendum

As for the other myths, the internet has made their meaning obvious. For thousands of years people wondered about the symbolic significance of, for instance, Cronos and his devouring of his children. Turns out it's just fetish stuff. All of it. We just didn't know people existed who liked that kind of thing.

Mythological Interpretation

Certain myths have the obvious purpose of explaining why a place or thing has the name it does. The Hellespont, for example. However!!?!?!! If that's the case, why do so many names not have myths appended? Do we just not have them? Were the sources of most names widely known or obvious similar to the approximately 5,000 places called Newcastle after translation? The likeliest cause is that the Hellespont was actually named after or at least by somebody a persuasive mythographer didn't like and wanted to erase. The second-likeliest is that mankind came here from another world and carried over most names from there but needed a few new ones. Third-likeliest is that all such myths, when put together and interpreted, tell us where the treasure is, and that's the one on which I'm banking.

Fashion Acknowledgment

It would have been far trendier to give a certain champion enchanted greaves instead of boots, but did he strike you as a trendy person? That's something writers call "characterization."

Against the Four Gates

King Ploutimos gathered up riches from his various conquests and stored them in his citadel at Hosses. He made no secret about it but rather boasted.
"And he who can win entrance can have the whole of it, but it will be too hard for any of you. For the wall has four gates through it. The northern is of iron and is guarded by Kyromedes whose helmet blunts every weapon. At the southern which is of bronze I have set Proimenos whose boots impart such deftness of movement that the ignorant swear he can fly. The west, which is silver, has at it the wielder of the dagger which when sheathed becomes coated in a poison that afflicts every man and monster equally, who is Phaimander. The golden gate to the east is defended by Archeuros, and to speak of him and his spear is useless, because none will challenge him."
The bold and the desperate made attempts on this gate or that, and many were the deaths and shameful retreats that resulted till Telephas son of Diophas came and laid Kyromedes low. He did not enter the citadel then but instead took the helmet and wore it against Phaimander, whose dagger then slew Archeuros, whose spear soon impaled Proimenos.
Even then Telephas did not enter but rather left Hosses entirely, saying he had no means to transport wealth where he had need to go, and that others might enjoy it as they wished.
Finis

The Key Role of Different Languages in Fantasy

When there's a poem, song, or convoluted prophecy, put it in a different language so that you can present a stripped-down prose translation to the readers instead of actually writing it.

On Height

When Apollodorus (Pseudo-Apollodorus if you want to be mean) declared Heracles to have been four cubits tall, he presumably did not intend to make that legendary figure a target of ridicule. Yesterday's giant is today's manlet it seems. For that reason, be sure to describe your hero in this way: "He was tall and handsome by the standards of your time and place." There, insulated. If you don't want your hero to be tall, yes you do.

Addendum

I presume we'll already have magic by then. If not, we should start with dwarves.