On extensibility, genuinely open to hearing what you'd want. What would "extensible" look like for your workflow specifically?
As for extensibility - I understand that your focus is system design, but I don't really see any reason for this to not be more broadly available for any sort of process design, particularly for use as a UX tool or in the product ownership space for understanding how systems and physical processes might be interconnected. I see that you can add any custom node in the code, and they just get a little gear icon, but it would be nice if the user could define the nodes themselves, add meta data, conditionals, loops etc. I've been using BPMN for years but I'd be lying if I said it wasn't outdated and a pain to present to people.