General antonyms:
* Originating
* Innovating
* Creating
* Inventing
* Developing
* Originating
More specific antonyms:
* Being original (if imitating means copying someone else's work)
* Being authentic (if imitating means trying to be someone you're not)
* Leading (if imitating means following someone else's lead)
* Being independent (if imitating means being dependent on someone else's ideas)
The best antonym will depend on the specific situation. For example, if someone is "imitating" a famous painter's style, then "being original" would be a good antonym. But if someone is "imitating" a friend's behavior, then "being authentic" might be a better choice.