With the proliferation of IoT devices, there is a demand for technologies to support high-velocity, dynamic resource provisioning to provide secure, cost-efficient and real-time IoT services in resource-constrained environments. Conventional fog computing by itself cannot address such requirements and needs to be complemented with opportunistic fog computing, by providing mobile fog resources on-demand. In this paper, we discuss key issues in this area, and investigate potential solutions from existing work. We conclude the paper with a summary of gaps, and propose an opportunistic architecture for future work.