Satisfying heterogeneous Quality of Service requirements of different applications in 5G systems requires tight interaction between the network and the applications. Although being pushed to 3GPP, SAND, the most mature solution enabling such interaction and cross-layer optimization for video streaming, has many limitations and drawbacks. This paper fills the gaps found in the SAND specification and enriches its applicability, by introducing a new general platform called xStream. xStream enables communication between various applications and the 5G system and opens the door for many innovative solutions for various types of traffic as well as optimal resource slicing (not only video ones). With extensive simulation, it is shown that xStream provides benefits for both OTT service providers and network operators significantly improving QoE for their customers.