From EN World, via Reynard (who is paraphrasing James Jacobs):
It is also worth noting that many RPG products, long and short alike, are consumed primarily as reading material. That goes not just for “fluff” too. James Jacobs has stated numerous times that APs are designed the way they are because they are primarily purchased by people that read them, not run them. Your audience wants an enjoyable reading experience and that includes the visual design of a product.
The takeaway? Don’t worry too much about producing 5E mechanics. If you’re inspired by something in a Volo’s Guide, or you see a way to link your work to a new mechanic, then write the mechanic.
Otherwise, focus on the story.
Let the story be the soil in which ideas for new mechanics grow. This was the core idea behind 3E’s Prestige Class concept, and you should do everything you can to marry story and mechanics going forward.
That’s what will sell your content on the DM’s Guild.