General Antonyms:
* Courtesy
* Kindness
* Generosity
* Consideration
* Respect
* Freedom
* Choice
* Option
Specific Antonyms (depending on context):
* Request (if imposition refers to an unwelcome request)
* Invitation (if imposition refers to an unwelcome intrusion)
* Benefit (if imposition refers to something burdensome)
* Help (if imposition refers to something unwanted)
The best antonym will depend on the specific context in which "imposition" is used.