Open source Excalidraw alternative

The open-source diagramming space has roughly 12-18 active projects competing for mindshare, ranging from full-featured tools like Draw.io and Miro to lightweight forks and experimental prototypes. The core pain points center on three failures: bloated UIs that slow down sketching, lack of offline-first workflows, and vendor lock-in through proprietary file formats or SaaS-only deployments. The real gap isn't in features—it's in the experience layer. Excalidraw wins because it feels like drawing, not using software. Most competitors pile on collaboration, real-time sync, and enterprise features, missing that founders and teams actually want speed and simplicity first. The opportunity sits in either dominating a specific use case (UX mockups, architecture diagrams, flowcharts) with a laser-focused tool, or building the infrastructure layer that lets other tools plug into an open ecosystem. A new entrant should focus on either: (1) extreme ease-of-use for a narrower audience (e.g., technical documentation for engineers), (2) better offline-to-cloud sync that doesn't betray user trust, or (3) building an extensible platform where third-party vibecoders can add their own diagram types without forking the codebase.