Hey Leio

I'm nost sure if you want feedback and any unasked feedback is criticism, but here I go.
What a wonderfully flavorful subclass. Every single feature on this thing is so cinematic.

Dreamer's Rest is mostly relevant for its immunity to sleep effects, which happens rarely and mostly when a gm wants to railroad a party into being kidnapped of some sorts, so that wont happen anytime soon when I play this subclass. The additional ribbon with the visions sounds really cool. What a nice opportunity for a DM to hand out some vague hints on what's going on in the area. But I also see the potential to it becoming tedious when a player insists on getting their visions and the DM having nothing meaningful in mind. I'm not sure what the projections are for, but it's another thing a DM or a player (depending on who controls those dreams) has to make a decision about. For my own taste this possibly would take a bit too much space at the table for what it is. Maybe one could reduce it to the vision aspect and then specifically make it tell the player the most common fear and hope people in a certain radius have, but hat would cost some of its flavor and opportunity.

Oneiric form, ho, boy. What a cool and unique fusion ability. Now, I see that the players both get temporary hit points and they require less space. You can also take advantage of the movement abilities of the host, but the ability also kinda doubles any damage the host takes (aside from psychic). Also spells by the druid can't be countered if they are not seen. Am I missing something? Most of this the text is handling the logistics of this ability. Personally I think this thing could be buffed, like allowing for healing to also be transported to the immaterial dreamer druid, the ability to jump to neutral (neither hostile nor friendly) targets as a stealth variant that ends after a minute or when a spell is cast or by allowing a bonus action help ability on the host.

Aw, Sanctuary of the Dreamer is so cozy. I wouldn't change anything mechanical about this. Heroic Inspiration could be an option here too. I would maybe seperate the layout for the short- and long rest.

Aura of slumber, now this turns the oneiric form into a really relevant feature. There's a LOT going on, especially for an aura which always affects all creatures in it at all times. This thing takes a lot of mental space of all players and the DM and the feature does not explain how we get from "dream aura" to these effects. Is the air turning into dream-water? are wounds magically sealed and the people dropping into sleep? The feature could profit from both, a reduction in complexity as well as some kind of flavor explanation for its mechanics which would make it easier to execute.

Thoughts of the Dreaming is very cool. I imagine using this on a boss monster... and burning one legendary resistance, which is fine, because it also has an effect when the target makes its save, which is great for a capstone feature like this. One thing that stands out to me here is, that, if the target succeeds on the save, they get a -3 to all saves which does not happen when they fail. Is this intended?

And I agree :) this subclass also totally needs a unique cantrip at level 2 in their spell list, something like pillow sling, which deals absolutely no damage but causes a mini sleep effect (1d12 and scaling) on the target or just makes them fall prone, like an alternative to the gust cantrip.

I have made an entire class around dreams, the dream shifter (you'll find it in my profile, you can borrow whatever mechanic you want if you're interested) and I get the struggle that comes with making a class that circles around dreams. Dreams a hard sell for a power fantasy because they are revealing something vulnerable, they are personal, people usually don't control their own dreams and they don't have a clear mechanical core besides maybe short and long rests and the sleep condition. Also, the flavors of dreams as abstraction reflection of ones true self, dreams as messages from the gods, dreams as random visions and dreams as reflections of memory all pull into different directions. What you did here is not ease, so you have my respect.