To be honest I don’t buy into astrology, crystal energy, or natural oils. HOWEVER, the number of times I’ve perused my copy of ‘The Beginner’s Guide to Crystals’ to determine the correct stone to represent the magic of an item is too common of an occurrence to ignore. There is so much lore built around these things that I would be a fool to just ignore it. To start off this rabbit hole of fringe material, lets cannibalize astrology.
Zodiacs
In the real world, the characters of the zodiacs have their own personalities, strengths, and weaknesses. In D&D we have several fun ways to create a unique zodiac structures. There are 8 schools of magic, 12 classes in the phb, and 14 creature types just to list things that immediately come to mind.
Once you’ve determined a zodiac you want to fiddle with you can begin brainstorming ways the zodiac affects your game. Maybe, if we use the creature type one, monsters of the specific sign receive a small bonus during their month. Maybe a zodiac effects the moods and behavior of creatures in the world. You could award inspiration or role-play xp to players who play out the effects of their sign.
Effects of the zodiac may actually have an effect the players plans. After all, it would be stupid to attack the red dragon while Draco is manifesting.
Astrological Bodies
As much as I love the abstract concepts of zodiacs it is nice having actual stars in the sky and making them a bit more magical. Having the star of the god of nature go into retrograde to show that they’re dying, having the constellation of the god of death disappear from the night sky to show that they’ve decided to walk the mortal realm, or maybe portray a shift of the god’s allegiance by moving their star closer to another. And that is just the storytelling stuff available to you.
Maybe certain stars are only visible certain times of the year and they do weird things to magic. Stars are actual creatures fighting off the aberrant darkness. What happens when the stars start dying and the darkness comes?
Thats all without even touching the moon, lunar cycles, comets, black holes, or any of the countless other cosmological things we can fiddle with. The point is, don’t disregard the use of real world myths in your game. You may be missing out on a metric ton of great material for your game.