The heterogeneity of the requirements for 5G necessitate a versatile 5G radio access network (RAN); slicing offers a way of realizing a flexible RAN through customized virtual subnetworks. In this article, we focus on how enabling lower-layer flexibility in the RAN affects the development of RAN slicing, particularly in relation to ensuring isolation between RAN slices. We first examine how RAN slices may be individually tailored for different services. We follow this up with an examination of the potential time-frequency resource structure of the RAN, focusing on the trade-off between flexibility and the overhead related to ensuring coexistence between contrasting RAN slices. Based on this analysis, we suggest an approach that permits the allocation of resources to a service-type to be performed separately to resource allocation for individual services belonging to that type.