Diagramming model - PlantUML is the first generation target. mxGraph is the next - option to specify layout and merge updates with manually laid out diagrams - match by ID (URI digest), update only the semantic part if found.
Delete if no match, some primitive layout for new - below all or similar to text wrapping - grow proportionally. First, validate compatibility of mxGraph-produced output with draw.io - mxGraph is end of life, but it is ostensibly used by draw.io
If no compatibility - a primitive XML-based output - don't need much functionality.
Longer-term potentially - diagram editing plug-in for Eclipse.
Two levels of draw.io support: a) Token expansion on hand-crafted diagrams - decompress, expand tokens, re-compress and embed - handle at the core. b) Generation of (initial) diagrams and merging of changes - handle at its own product, try to leverage mxGraph.
Support of Mermaid output.
Nasdanika