The Syncytial Mesh Model introduces a three-layered framework for large-scale brain dynamics integrating local neural circuitry, macrostructural connectivity, and a slow mesoscale control-field substrate associated with astrocytic syncytial organization. Rather than directly generating electrophysiological activity, the proposed syncytial layer modulates neuronal excitability, coherence structure, and metastable coordination across spatial scales. The framework is formulated as a phenomenological effective theory combining neural-mass dynamics, connectome-scale coupling, and continuous-field interactions. Within this architecture, the model provides a candidate explanation for large-scale traveling-wave organization, low-frequency coherence structure, and distributed plasticity phenomena that are not straightforwardly reducible to direct local synaptic connectivity alone. The model further generates experimentally testable predictions concerning astrocytic modulation of large-scale coherence geometry, developmental maturation of low-frequency resonant structure, and pathological alterations of mesoscale synchronization in disorders affecting glial connectivity and calcium dynamics. The Syncytial Mesh Model is proposed not as a replacement for existing neural-field or connectome-based approaches, but as a complementary mesoscale framework for understanding large-scale coordination dynamics in the brain.