General Antonyms:
* Diverse
* Varied
* Heterogeneous
* Fragmented
* Decentralized
* Modular
* Multifaceted
* Multi-layered
* Polycentric
Antonyms related to structure or organization:
* Decentralized
* Distributed
* Dispersed
* Modular
* Segmented
* Structured
* Hierarchical (though this can also be viewed as a form of "monolithic" in a different sense)
Antonyms related to ideology or perspective:
* Pluralistic
* Eclectic
* Multi-faceted
* Open-minded
* Tolerant
* Flexible
The best antonym will depend on the context in which you are using the word "monolithic."