Basement with tile floors. Take out the entire kitchen, fill a wall with random mirrors, add a barre for ballet. Have a little drawer for little stuff like pointe shoes, maybe some tutus, hair ties, and to hold a speaker for music.
The hall of the main library in Prague is dominated by a giant tower of books - a monument made out of old novels from the library. It rises more than 5 metres in the air. In the bottom, there is a mirror, creating the effect of a deep, deep well of wisdom.