As a fundamental component of GNSS signals, messages act as a critical medium for transmitting information required for PNT, serving the needs of both service providers and users. Over the years, message structures for GNSS signals have evolved from fixed formats to pseudo-packetized and mixed formats. This evolution has facilitated the integration of supplementary data and has driven further research to incorporate new types of information. These developments have highlighted the necessity of flexible and transmission-efficient message structures. In this paper, we propose a set of performance metrics designed for the comprehensive evaluation of GNSS message structures. Using these metrics, we analyze the performance of existing message formats. From the results, it is observed that optimizing message formats based on the purpose and characteristics of the transmitted information could achieve flexibility and transmission-efficient. Based on these findings, we propose a novel approach to designing message structures that address future requirements.