Considering the overarching trend in mobile communication development, the extensive coverage and seamless connectivity offered by non-terrestrial networks (NTN) are becoming increasingly essential. NTN communications, characterized by extensive data volumes, hyper-connectivity, and large-scale fading, present notable challenges to traditional communication methods. This paper proposes a semantic communication-based NTN (SC-based NTN) architecture, highlighting its potential applications. However, the integration of semantic communication within NTN faces three key challenges, including multimodality, security, and resource allocation. This paper explores the opportunities presented by artificial intelligence, physical layer security, and intelligent resource management technologies, which promote multimodal, secure, and efficient communications for NTN. Finally, the paper outlines a visionary outlook for SC-based NTN, highlighting its potential impact on the future of mobile networks.