Hi, I've been a fan since before the fairy faction was added but didn't have an account. I made one so I could post comments on this game specifically because it's my favorite on this site.
I looked into the progress menu to see what gaps existed between actions, and there were tons! Even spots where I would never have guessed there could be a gap for more actions to be placed, there are, so there's a lot of future proofing. Some of the gaps are clearly for organization or were just part of the dev process and aren't necessarily going to be filled with new content, but in looking through the gaps I thought of a few ideas.
(Similar to other comments, I just want to lay these out as ideas, anything or nothing can be done with them. It's your game, after all.)
There's room to expand on the [Spoiler for v18] action that could be fun, but possibly for endgame or an alternate ending, given how it warps the gameplay loop of the game to pursue it.
There's a gap for an expansion to farming in the starting village, though this might also be for the training dummy, perhaps stronger materials can lead to a stronger training dummy.
A more personal idea is somehow the rats from the tavern leading to a sewer area somewhere, though I don't know how that would work or where that would be. (Maybe in the City? IDK on this one)
The gap that I believe was where the Fairy faction was meant to be placed initially (but was too small) could instead be used for a small logging expansion, not a new skill, just more wood or types of wood, maybe a lumberjack cabin.
There's a gap between building traps and catching rabbits that I think building stronger traps of stronger materials for catching stronger or rarer creatures would fit in nicely. The next gap could be used for rare creatures that are harder to catch than rabbits.
There are two small gaps that might be useful for an expansion to the Wolf faction, the second of which being more usable since it's placed after the faction has been allied with. That one is up to you entirely, I'm not sure what to do with that.
Goblins have several gaps, there's one for expanding the training camp (maybe by using the various goblin types), training more types of riders, potentially from the rarer caught creatures, more explosions for the Goblin exploders, (I don't know if itch has an issue with the word of the object that causes explosions, but I'm playing it safe,) more goblin types, and a large space for other Goblin upgrades.
The Fairy faction has several gaps as well, there's one for possibly purifying other things or other early things to do with the Fairy, then there's one for fighting other corrupted creatures, then there's a decently sized one for more Fairy upgrades.
These ones are the ones I find the most interesting, in between offing the fairy and creating the potions there's a gap, and between creating the potions and consuming the potions there's a gap of exactly 5 slots for actions, one for each potion.
Now, you can do whatever you want with this, maybe nothing at all. But, given that the Fairy faction has no direct opposite or rival faction currently and the fairy wings and the potions they're used for are not as strong as sparing the Fairy, maybe this could be an opportunity for an enemy to the Fairies, a magically inclined faction who would love the Fairy Wings or the potion the player brews with them. Maybe a Witch/Dark Fey faction? This is my strongest idea honestly.
Going further to the Elves there are just so many gaps for things, enter the village, there's a gap for more stuff to do when first entering the village, buy the bow, a lot of room for archery upgrades besides crafting arrows, then there's a gap for plenty of arrow types, which I'm sure is a trend that'll continue.
There's a massive gap between the Elves and the City, enough to fit an entire area as big as the Forest, with multiple factions. This could be a split off from the Forest in a different direction from the City, or a magical dimension entered through one of the factions in the forest somehow. There are infinite possibilities and they're all yours. Don't just stop at my ideas.
There's room for more books, there's room for other things to do in the library, there's room for more things to buy at the market, there's room for more merchant related actions, possibly getting into the merchant business for reliable gold or some other quest. There's room for faction related forging, and there's a decent bit of room for a blacksmith faction? This wouldn't be a typical faction, more like the Wolves, smaller in scale. I was thinking they would be rivals and that the Dire Wolf pelt is how you would access the true depth of the Blacksmith faction's actions. Again, that's just my idea, do whatever you want with your many gaps between actions.
There's a lot of room to expand the tournament and the rest of the City. More travel locations from the City could be added, though each one presumably would be its own area, so I can see why there's only a few open slots.
There's a large gap between travel and the Holy faction that I'm not sure what would go there, but that just means it's a blank canvas.
The Holy faction could be expanded, being evil could be expanded, and there's a large gap between this and the mountains where anything could be added.
There's a gap for actions between reaching the mountains and entering the mine, more things could be done there, more action can be done in the mine before fighting the crystals, more monsters can be fought in the mine, maybe, more ordinary ores could be mined, like gold or copper, maybe for use by the merchants or other factions.
More stuff could be added past the Dwarves and Dragonkin in the cave location-wise, just go past there settlements and see what's further past them. Just a thought.
More stuff could be added with the Sage, maybe fighting other magical beings like summoned creatures or training with them, (IDK what the Sage is, so I apologize if I got it wrong,) there's a decent gap that could fit the mountain's peak being expanded with more actions, that's the area where the yetis are.
More starting actions could be added to when you first get to the swamp, there could be other Necromancer upgrades later on that don't involve the zombies, (adding more or strengthening them,) and instead strengthen the caster, there seems to be a spot for that type of upgrade. There could definitely be other stuff added to the swamp that's unrelated to the necromancy or mutagen later on, and eventually a new area might be an idea worth considering.
Ultimately, it's up to you, and always make sure to pace yourself and avoid burnout by taking breaks if you start feeling it.
TLDR;
You're doing good, love the game! You future proofed it really well, I just wanted to point out how well you did that.
Some ideas: Witch faction to rival Fairies if you off the Fairy for wings to make potions? Blacksmith faction to rival Wolves if you off the Wolves for pelts to craft armor? Mining other ordinary ores like Copper, Gold, Platinum for use with interacting with other factions in trades? Sewer Area from Rats in Tavern, maybe in City? Magical Dimension Area from one of the forest factions?
All ideas are up for grabs, but no pressure. Just wanted to offer some possible ideas, no need to use it if you don't want to. Thank you for making a really fun game!