Ok, so to be honest, next game I don't have any ideas of what to do, and I came up with the idea of making a session inside the PCs dreams (A collective dream manipuled by someone, perhaps a god or something like that). The main plot have come to a moment that I need to hang it for a bit. What's the main plot? A meteor have fallen from the skies (actually from avernus) and inside of it there's nothing less than the book of vile darkness, delivered from Zariel to the BBEG. Zariel wants him to build an artifact to hold inumerous souls so she can use it as a "nuclear bomb" on the blood war. The BBEG however wanted the book, it was a bargain. Well, I will not get deep into this, but now that the deliver has been made already I need to fill the game with other stuff until something comes up and I tought about this dream. The purpose of the dream is that it is very crazy and full of meanings about what could happen in the future. I even intend to put up a "fake" fight with a future boss.

Do you have any tips or ideas on how to run sessions within a dream? I'm afraid the PCs will get frustrated that they're dying to the boss, or they'll get frustrated that their actions are being distorted and things happening without any sense (it's a dream after all). But, I also hope they suspect it's a dream, but not too fast (and not all PCs)

I don't have much, but I ran a session in an area where the landscape shifted randomly which might tip them off (walking in grasslands then 15 minutes travel and they are on a mountain slope, another half hour it turns to swampland, ect.)

I've run quite a lot of dream sessions.

First up, I'd be clear to the players that they're in a dream. Second, make sure that they receive the information that if they die in the dream, then they die in real life.

I'd start them off with something really bonkers - my players were hallucinating after eating some shrooms, so they appeared in a world of psychadelic mushrooms and flumphs. Then the dream turned bad, and they went through a series of scenarios that were related to their pasts, including some things that they didn't know. The cleric saw his parents and learned a little of his back story, the barbarian observed some of where his wild magic came from (he didn't realise it at the time), the fighter found himself inside the pages of a book, and the Order of the Profane blood hunter found saw the demiplane where his great old one patron was imprisoned. The players had to puzzle or fight their way through each scenario.

The advantage of this is that it made the dreams important to the individual players, whilst still existing only in a dream world.

I think Sanvael is onto something with layering in PCs backstory elements.  If you know your particular PCs fears or phobias that can be helpful when the dream turns into a nightmare. 

I once listened to a sleep researcher who talked about the lack of permanency in dreams.  For instance, if you're looking in one direction in your dream and turn your head to look at something else, when you turn your head back to the original direction what you see is often something completely different (because its all in your head).  You could use this mechanic to change the setting quickly - rooms or places behind closed doors that shouldn't exist, an NPC calls them from behind and when they all turn around they are in a completely different location, diving into a pool of water to swim to the bottom only to find yourself flying upwards to the clouds when you breach the surface, weird things like that.

Dreams are based on reality and memories, but distorted. It's also possible for your dream to be affected by the environment around you. When you sleep, anything you hear can influence your dreams. For example, I once programmed my TV to turn on on a specific channel and time to act as some kind of alarm clock. During the morning, I dreamed of a tornado destroying my rural neighborhood. I was basically inside a disaster movie. Although it wasn't my first dream of this genre, as I've had recurring dreams about volcanic eruptions and artillery fire before, it was the first one with a tornado. I've also never lived in a rural area. Pretty much everything about this dream was original, and I never repeated it. When I woke up, I saw that a documentary on tornadoes affecting rural areas was playing on my TV.