General emptiness:
* Empty
* Vacant
* Hollow
* Void
* Blank
* Bare
* Desolate
* Depleted
* Unfilled
* Lacking
Emptiness as a feeling:
* Loneliness
* Isolation
* Despair
* Hopelessness
* Meaningless
* Pointless
* Devoid
* Lost
* Alienated
Emptiness as a state:
* Silence
* Stillness
* Quietude
* Apathy
* Indifference
* Zen (in the sense of mindful emptiness)
Emptiness as a lack of something specific:
* Heartless (lacking compassion)
* Soulless (lacking spirit)
* Mindless (lacking thought)
* Purpose-less (lacking direction)
* Contentless (lacking substance)
* Uninspired (lacking creativity)
Emptiness as a concept:
* Nihilism
* Existentialism
* Nothingness
* The abyss
Figurative emptiness:
* Ghostly
* Ethereal
* Spectral
* Vacuumed
* Drained
More specific words:
* Vacuous (lacking intelligence)
* Futile (useless)
* Inane (silly)
* Sterile (lacking life)
* Abject (miserable)
The best word to use depends on the specific context and what kind of emptiness you are trying to describe.