If "initial" refers to the beginning:
* Final
* Last
* Concluding
* Ultimate
* Subsequent
If "initial" refers to the first part or stage:
* Final
* Concluding
* Ultimate
* Subsequent
* Later
If "initial" refers to the first letter of a word:
* Final (referring to the last letter)
Example:
* Initial reaction: The antonym could be final reaction.
* Initial stage: The antonym could be final stage.
* Initial letter: The antonym could be final letter.
It's important to consider the specific meaning of "initial" in your context to choose the most appropriate antonym.