This study evaluates how co-branded packaging design can shape consumer purchase intention toward time-honored (heritage) brands by synthesizing recent branding and packaging scholarship with qualitative insights from semi-open interviews. Co-branding has become a prominent rejuvenation tactic for heritage brands, yet collaboration outcomes remain mixed because packaging, often the most immediate touchpoint at the shelf and in digital storefronts, can simultaneously signal novelty and undermine authenticity. Drawing on a semi-open interview guide and interview transcripts from 20 participants, the study applies reflexive thematic analysis to identify (1) opportunity pathways through which co-branded packaging increases purchase intention (e.g., attention capture, perceived rejuvenation, giftability, and social shareability), and (2) limitation pathways that constrain or reverse these effects (e.g., perceived lack of fit, authenticity dilution, collaboration fatigue, price-premium resistance, and sustainability concerns). Findings suggest that purchase intention is most likely when co-branded packaging achieves a "heritage-anchored novelty" balance: it retains recognizable heritage cues while integrating partner elements in a coherent narrative and value proposition. Conversely, when packaging cues appear superficial, overly commercial, or environmentally wasteful, consumers activate persuasion knowledge and discount the alliance. The study contributes an integrated mechanism model linking packaging-level stimuli to consumer evaluations (fit, authenticity, novelty, value, trust) and to purchase intention (trial, repeat, gifting). Managerial implications are offered for design governance, collaboration cadence, and price/value communication in co-branded packaging for time-honored brands.