Formal:
* Dans vos rêves: This is the most literal translation and is appropriate for formal situations.
* En rêve: This is a more literary way to say "in your dreams" and is also appropriate for formal situations.
Informal:
* Tu rêves: This translates to "you're dreaming" and is a more casual way to say "in your dreams." It can also be used as a way to tell someone that their hopes are unrealistic.
* Rêve un peu: This translates to "dream a little" and is a softer way to say "in your dreams." It can be used to encourage someone to have big dreams.
Sarcastic:
* C'est pas demain la veille: This literally translates to "it's not tomorrow the day before" and is a sarcastic way to say "in your dreams." It implies that something is unlikely to happen.
* Tu peux toujours rêver: This translates to "you can always dream" and is another sarcastic way to say "in your dreams." It suggests that something is impossible.
Other options:
* Dans ton sommeil: This literally translates to "in your sleep" and is another way to say "in your dreams."
* Tu te fais des illusions: This translates to "you are making illusions" and is a way to say that someone is unrealistic.
The best way to say "in your dreams" in French depends on the specific situation and the message you want to convey.