Figuring out what to publish as a supplement is always a bit of a puzzler. You see, what sells are corebooks, which was the philosophy at WOTC during the 4th Edition days, and which has bled over somewhat into 5th Edition publication. They say they’re moving away, a bit, from the heavy hardback tomes for everything, but it remains to be seen.
Why have these big tomes become the norm?
Well, supplementary material can be a hard sell. Let’s say you have a GM’s book and a Player’s book.
In practice you’re going to sell one GM book per group (typically 4-6 players) and maybe slightly more Player’s books. That’s still only about one sixth your potential audience, and that’s the most you’re going to sell of just about anything.
Once you get into supplementary material, class books, race books and so on, you’re carving that fraction down more and more. Even the best possible market is only a fraction of the number of players.
What has traditionally sold worst of all (apart from during the early days of gaming when people were starved for material) has always been adventures.
Because adventures tend to be one-and-done. You can’t very easily play through the same adventure again, especially not with the same players. Reusable materials might be a couple of interesting traps, some monsters and some magical treasure, but it’s not convenient to flip through dozens of adventure books to find the material you want to re-use.
If you want to sell supplements they need to have a purpose and some longevity.
More character options.
Expanded systems for new circumstances.
Otherwise, you’re in for a bad time.