The vision of the Internet of Thing is becoming a reality and novel communications technologies such as the upcoming 5G network architecture are designed to support its full deployment. In this scenario, we discuss the benefits that a publish/subscribe protocol such as MQTT or its recently proposed enhancement MQTT+ could bring into the picture. However, deploying pub/sub brokers with advanced caching and aggregation functionalities in a distributed fashion poses challenges in protocol design and management of communication resources. In this paper, we identify the main research challenges and possible solutions to scale up a pub/sub architecture for upcoming IoT applications in 5G networks, and we present our perspective on systems design, optimisation, and working implementations.