The telecom industry is striving to evolve the current 4G system to the 5th generation of mobile networks, aiming at boosting data rates and performance for the traditional broadband services, as well as supporting a wide range of new use cases with different technical requirements. The first specifications of the 5G system are included in 3GPP's Release 15 and expected to be enhanced in the future Release 16. In order to cater for the diverse network requirements posed by the 5G use cases and in the view of instantiating customized networks that suit the vertical industry, a programmable and flexible data plane is envisioned as a key element of the future system architecture. In this context, this paper presents an advanced core network design underpinned by SDN principles, introducing enhancements to the current mobile networks' functional architecture and leveraging programmatic approaches for the agile provisioning and re-configuration of customized data planes. Our proposal is substantiated and validated by means of an initial experimental prototype, carrying out a functional proof of concepts including commercial 4G cellular base stations and devices.