It’s probably not the most poetic, but I like it:
Luftschloss
It literally means „air castle“. It’s a castle made entirely out of air. It looks stunning and grand in your imagination or when you talk about it, but it’s not real.
It’s like having a dream or a vision that sounds great but is completely unrealistic or impossible to achieve.
In English, you might say „pipe dream“ or „castle in the air“, but „Luftschloss“ sounds way more poetic and fragile, evoking something beautiful yet intangible.
There’s another one which is similar, but not the same:
Hirngespinst
The literal transition would be a delicate web spun by your mind. In Englisch you’d probably also say „pipe dream“, but there’s a subtle difference between „Luftschloss“ and „Hirngespinst“.
„Hirngespinst“ is more like an absurd or even silly idea you came up with. By contrast, a „Luftschloss“ tends to describe a beautiful but unrealistic vision with a broader, often more hopeful scope.