What i like about the KIP (and PEP, RFC) is that they are examples of successful “Design documents” from complex Open Source projects. GitHub issues and PRs are good for isolated well defined, changes, but they may prove lightweight for larger design work and formal decision making / voting.
A KIP allows for a consistent, thorough design, making sure all options are considered, that public APIs are not changed without a consistent plan, that security and performance is always a topic etc. And it provides a public record of important project decisions. A single approved KIP may lead to multiple PRs across several repos. If you look at Python, PEPs are the main decision making and recording vehicle in the project, where PEP-1 defines the PEP process itself, PEP-13 even defines the governance of Python etc.
Anyway, I lifted it here mostly for inspiration and to start thinking about how to solve the roadmap/design problem in a true, distributed and open fashion.