On country-specific tax logic and edge cases: I’m planning to design the core with extensibility in mind—so things like tax rules, invoice sequencing, and regional compliance can be handled via plugins or configuration modules. For example, German invoice sequencing or Singapore GST could be implemented as optional adapters, and I’d love help identifying the most common patterns and pain points.

Authentication: Unified auth/session handling is a big goal, but I recognize each platform’s OAuth flow is a bit different. My idea is to abstract the common flows and provide helper functions, but also allow for custom overrides where platforms deviate too much. I’d be interested in hearing more about where OAuth gets especially tricky for your use cases.

Data mapping/sync: This is definitely the hardest part. My current thinking is to define a canonical data model (for invoices, customers, etc.), then provide mapping layers for each platform. There will be tools to help with syncing and conflict resolution, but I expect this will need ongoing refinement as APIs evolve and edge cases pop up.

Would love to keep this conversation going—if you have specific war stories or must-have features, please let me know! And if anyone’s interested in collaborating, I’d be happy to open up the repo early for feedback and contributions.