So what am I doing with Ynn, anyway?

It's somewhere between a module and a setting. A sub-setting that you can bolt onto your game, I guess.  

The idea is that you've got a bunch of random tables to produce an encounter/location generator. Just pick it up, roll dice, get something to throw at your players. Kinda procedural generation as you go along. I really like modules that do that (Castle Gargantua and Scenic Dunnesmouth come to mind, as does Corpathium), for a couple of reasons.

Firstly, a module that proceduraly generates content has a load more replayability to it. The different combinations in encounters mean that although there'll be a consistent 'feel' to encounters and locations, you can go ages before it gets old. In Wolfpacks, for example, the basic terrain/cave tables produce enough content to run a campaign off by themselves. Compare to a module with 'set' encounters written by the author, where you play through maybe 20 to 50 scenes and then your done. (Of course, the advantage of 'set' encounters is in the pacing and links between encounters, and the inividual details).

The second is in how you actually use the module at the table. In my experience, a GM's comprehension of stuff somebody else wrote will be sub-optimal. When you're frantically skimming the  module at the table, you're gonna miss bits. Whereas if you wrote something yourself, because it's your creation, you're gonna grok it. Random generators are good because they're a tool for GM creativity. They take the pressure to 'quick think of something cool right now' off the GM by providing concepts, but let the GM put them  together themselves. This has the advantage of fixing the ideas in the GMs mind much more easily.

I want the feel to be kinda fairytale-ey. A bit surreal, a bit liminal.
One idea that always stuck with me was the 'wood between worlds' from the narnia books. That place was always written with this weird dreamlike quality that makes you want to explore further. I like that.
The castle from the Ghibli film 'Laputa, Castle in the Sky' is a big influence here. Ruined architecture, magitech still working, slowly falling apart, even with all the inhabitants gone. Lush nature in a garden left to go wild.
Alice in Wonderland is in there a bit. Mostly via A Red And Pleasant Land. But focussed on the whimsical ideas more. The tea-parties and ambulatory puddings rather than the vampires doing atrocities.
I'm liberally nicking ideas from The Stygian Gardens Of Aemelia Premm. It's a good module.
Made In Abyss continues to mull around in my head. The idea of going 'deeper' into a naturalistic planty setting with abandonned structures came from there.
I want there to be whimsy. I want it to be comfy and chill but weird and unsettling. My standard medieval-fantasy game is LotFP (with a few houserules taken from Corpathium), but while the engine is cool this module's tone is gonna be very different. None of the grimdark and shock value.

I'm using old public-domain illustrations to illustrate it. Mostly from fairytales. There's some absolutely wonderful Arthur Rackham pieces that will work wonderfully with very little editing.

I'm planning to use Sidhe in this. Not quite elves, not quite aliens. Art-nouveau aesthetes with magitec plant powers. They're almost entirely gone now. Those that remain have retreated into solipsistic feral decadence. But, their presence lingers, their mark on the place is indelible.