I'm running a modified version of the Tomb of the Nine Gods, where I explained that the tomb had been altered by the beholder within. One of the players has come up with a plan to cast Dream on it, in an attempt to reshape the dungeon. Any ideas?

There's a lot of room for this to go horribly, horribly wrong for the players. If a beholder dreams about a beholder it spawns one into the real world outside of the dream. So, if everything goes well they could maybe pull off reshaping the dungeon. If it doesn't go well they could end up with two beholders to deal with.

A beholder is always half awake, or at least not all the way asleep, as at least one of its eyestalks is awake and alert at all times. This lack of proper sleep is actually one of the main things that drives a beholder insane. So Dream might not even work. On top of that, there's nothing in Dream that says the target can't just wake up. A sleeping beholder is already prepared to wake up and spring into action at the drop of a hat, so unless the players prevent it from figuring out a spell has been cast on it, it can just wake up before anything bad happens. If the players avoid that, and you rule that Dream actually works... hoo boy, you have a fun encounter on your hands. The spell states that the messenger actually appears in the target's dreams, meaning it appears in physical form. Guess what that means? Whatever form the messenger takes on in the dream will appear in real life. As the messenger has control over its dream form, it basically has a cool minion for as long as the spell lasts, or maybe even after it ends, it's really up to you what you do with it, if it's not killed. Also, the players aren't the only ones who can conjure beings into existence. This is the beholder's power they're hijacking, after all. Anything they do might cause it to dream up something in response. One might imagine that the beholder actually reshapes its surroundings whenever it dreams, so it could have something in the dream that actively reshapes the surroundings into what they once were unless it's destroyed or the beholder wills it otherwise.

Quote from KLM668 >>

I'm running a modified version of the Tomb of the Nine Gods, where I explained that the tomb had been altered by the beholder within. One of the players has come up with a plan to cast Dream on it, in an attempt to reshape the dungeon. Any ideas?

 Basically you're having a duel inside the beholders dream with both of you able to control your surrounding. Get ready for a lot of charisma and intelligence saves...

Have the spell fail. Make a side effect, here's a list of possible ones:

1. Have them sucked into a world inside the beholder's mind. They have to fight a bunch of mini beholders ([monster]gazer[monster]s) and other monsters inside its mind (quori, gauth, cave fishers, mind flayers, flumphs, goldfish, etc) , possibly some clones of themselves to fight. Maybe this fact isn't immediately obvious to them. They could very just appear in their previous lives with minor differences, like one of the NPCs from their backstory ends up being a gazer in a trenchcoat, or something like this, and they slowly realize that when they cast the spell on the beholder, they didn't escape the dungeon, but are instead stuck in its dreamscape and has to find someway to escape. 
2. They dungeon is changed, but not in the way the players want. Maybe the whole level is separated from the other levels, and they have to find a way to the lower levels from the newly moved level of the dungeon. (If you're up to really challenge your players, make the dungeon come to life as a huge stone beholder, which they have to stop from destroying Chult)
3. The spell Dream is cast on all of them, as well as the beholder. 
4. The character switches minds with the beholder. 
5. The beholder gets the character's spellcasting ability.