Decentralizing the governance of social computing systems to communities promises to empower them to make independent decisions, with nuance and in context. Yet, communities do not govern in isolation. Many problems communities face are common, or move across their boundaries. We propose designing for inter-community governance: mechanisms that support relationships between communities toward coordinating on governance issues. Drawing from workshops with 24 individuals on decentralized, community-run social media, we present six challenges in designing for inter-community governance surfaced through ideas discussed in workshops. These ideas come together as an ecosystem of resources and tools that highlight three key principles for design: modularity, forkability, and polycentricity. We end with a discussion of how workshop ideas might be implemented in future work aiming to support community governance in social computing more broadly.