The Disney BSDF [1, 2] is extremely flexible and offers a lot of artistic freedom. The only real limitation is that the BSDF is required to be energy conserving, but that's hardly a limitation. A physically-based renderer is still able to produce non-photorealistic results.

[1] http://blog.selfshadow.com/publications/s2012-shading-course...

[2] http://blog.selfshadow.com/publications/s2015-shading-course...