The next generation of mobile communications, 5G, is expected to enable global economic output of USD 12.3 trillion by 2035. This would be achieved by applying 5G networks not only to mobile broadband, but also to diverse use cases such as machine-type communications and ultra-reliable low-latency communications. The diverse use cases and requirements are best handled by network slicing, which creates virtual networks over a single physical network. In this paper, we provide an overview of the standardization landscape of network slicing and identify major challenges for mobile operators such as end-to-end scope, roaming, interoperability, and migration. We conclude that the mobile industry and academia should focus on virtualizing base stations and sharing wireless spectrum resources, while selecting set verticals for which to create standardized slices and to formulate relevant requirements, where the GSM Association is a major player to be monitored. The industry should also provide a resolution to ensure the interoperability of network slicing, and study measures to ensure the stable and seamless migration from the physical to virtual networks. With the challenges resolved, mobile operators will be able to harness the full potential of network slicing in their mobile networks.