The rise of 5th Generation (5G) based network systems provide the prospect for an unprecedented technological revolution in different aspects of current network infrastructures to fully satisfy the high demands of smart space. This work ad- dresses the challenges that raise in exploiting the potential of WiFi WLAN-sharing technology in 5G Ultra-Dense Networking (UDN) use cases. We investigate new complementary aspects of emerging 5G technologies such as Network Function Virtualization (NFV) and Fog computing to design a unique WiFi WLAN-sharing ecosystem to allow complying with 5G UDN critical requirements. In the resulting approach, we empower WiFi WLAN-sharing infrastructures with Fog computing capabilities and follow a slice-defined approach, aiming to provide differentiated services at unprecedented levels, on top of the same infrastructure through customized, isolated and independent building blocks. The solution also enables slices to accommodate applications besides networking functions, seeking to provide ultra-low latency rates by leveraging direct linkage to data producer entities. A proof of concept was conducted by carrying out experiments in a real laboratory testbed, allowing insights into the feasibility and suitability of slicing WiFi WLAN-sharing systems.