Many efforts in the area of computer security have been drawn to attribute-based access control (ABAC). Compared to other up-to-date models, ABAC provides more granularity, scalability, and flexibility, which make it a valuable candidate for securing collaboration between organizations, especially over an open network such as the Internet. On the other hand, this model lacks provisions for trust and privacy issues, both of which are becoming increasingly critical, particularly in collaboration environments. Recently, we proposed a preliminary model to address this gap. This paper is a further discussion and development of how trust and privacy can be incorporated in the previously introduced ABAC model. In addition, we propose a structure for access control model that can cover most of real world access scenarios and schemes. The paper concludes with some remarks on implementation of such models along with possible future directions on evolution of access control models in general.