This paper presents the design options for creating a Pan-European mobile network for research in the context of the European Horizon 2020 EuWireless project. The most likely direction is a platform that makes it easier to create network slices for research. In this context, we identify one promising technology to implement network slicing in 5G networks: the framework GÉANT Testbeds Service (GTS). GTS is currently a production service by GÉANT that offers remote construction and use of virtual testbeds for wired networks mapped to the real GÉANT infrastructure. These GTS-virtualized testbed environments conform to Software Define Networks (SDNs) principles and offer compute, storage, and switching resources, at scale and with line rate performance. In this paper, we explain how the current (wired oriented) GTS can be extended with the 5G components, such as radio access nodes (gNBs), transport networks, user devices, etc., in order to implement 5G network slices. Our first conclusion is that using GTS for EuWireless implementation is feasible, dramatically increasing the potential impact of this service in the research community.