空想

daydream / fantasy / to fantasize

empty / air / sky / in vain

to empty / vacant / unoccupied / space / leisure / free time

to think / to believe / to suppose / to wish / to want / to miss (feel wistful about the absence of sb or sth)

to think (about) / to think of; to devise / to think (that); to believe (that) / to desire; to want (to) / to miss (feel wistful about the absence of)