Modern cosmology assumes that spacetime on large scales is sufficiently homogeneous and isotropic such that the propagation of light over cosmological distances can be treated as occurring through a uniform metric. This assumption underpins distance inference, redshift interpretation, and the mapping between observed signals and cosmic history. This work proposes a conceptual framework—Regime-Dependent Spacetime (RDS)—in which spacetime may exhibit distinct effective propagation regimes depending on environmental factors such as matter density, gravitational structure, or large-scale void dominance. Under this hypothesis, local spacetime physics remains consistent with established theory, while ultra-low-density regions may exhibit subtly different effective metrics for signal propagation without invoking violations of local causality or known physical constants. The framework does not propose faster-than-light transmission or modification of local physical laws, but instead explores whether cumulative propagation effects across different spacetime regimes could bias cosmological distance inference, potentially influencing interpretations of late-time acceleration or large-scale structure observations. This paper is explicitly exploratory and non-mathematical, intended to clarify conceptual assumptions, identify falsifiable implications, and outline observational domains where regime-dependent effects—if present—would be constrained or ruled out. The goal is not to replace existing cosmological models, but to examine whether an implicit uniform-metric assumption warrants further scrutiny in the era of precision cosmology. _________ Version note: Version 2 adds a short section clarifying potential falsification paths and observational relevance. No substantive claims or conclusions have been altered.