General Antonyms:
* Open-minded
* Flexible
* Adaptable
* Progressive
* Innovative
* Liberal
* Unconventional
* Broad-minded
* Receptive
* Tolerant
* Imaginative
* Creative
Specific Antonyms related to specific meanings of "hidebound":
* Unorthodox (for "rigidly adhering to tradition")
* Forward-thinking (for "resistant to change")
* Experimental (for "unwilling to try new things")
* Unconventional (for "bound by convention")
The best antonym for "hidebound" will depend on the specific context in which you are using it.