Large-scale ubiquitous computing applications are rapidly emerging in the fields of pervasive healthcare, smart cities and so on. They present unprecedented challenges to state-of-the-art ubiquitous systems in the respects of accommodating fluctuating user demands, handling volatile data quality and adaptation to complex system and user contexts. Driven by a motivating scenario in future mega-hospital environment, we propose to exploit the potential of cloud computing in supporting large-scale ubiquitous computing applications. This position paper will present the novel concept of EUPaaS (Elastic Ubiquitous Platform as a Service), outline the key research topics, and propose a cloud-based ubiquitous application platform.