Over the years, knowledge on how to engineer software-intensive system-of-systems (SoS) have been expanded and advanced. With the emergence of smart cities, which is a remarkable example of SoS, there is also an emerging need that almost any type of system should be prepared to join the smart city system and become a constituent of that SoS, even temporarily. However, several attempts to build a real SoS have stumbled on the limitations of single systems, also known as Constituent System (CS), to fully interoperate in the context of a SoS. The majority of the existing systems were not designed to be part of a SoS, interoperate in a instantaneous way, or maintain their operational independence despite their participation in one or more SoS. Based on this context, the main contribution of this paper is to exploit how to design a software architecture for systems that are intended to become part of a SoS in the future. We conducted a software architecture design process to conceive a constituent architecture for Urban Mobility domain as CS. We evaluated the architecture by means of simulation. Preliminary results reveal that our proposal comply with the requirements raised during the architectural design process and can increase the possibility of a system to be a constituent of SoS.