Pie in the sky
Definition 1 of 1
Pro Tip 1/3Unrealistic Optimism
Refers to expectations that are highly unlikely, often ignoring practicalities or realities of a situation. 
He's dreaming of a promotion without any work experience—total pie in the sky.
Pro Tip 2/3Overly Idealistic
Indicates that someone is naively hopeful, not considering the likelihood of success. 
Her idea of free cars for everyone is a nice thought, but pie in the sky.
Pro Tip 3/3Disappointment Potential
Can imply that the belief in such a plan may lead to disappointment when it doesn't happen. 
They planned to win the lottery for their retirement fund—just a pie in the sky idea.