The apex of word processing applications for writers.
Potentially extended LibreOffice? It appears an SDK exists and API examples from several languages.
From the docs: The SDK provides a rich set of examples in different programming languages (Java, Python, C++, LibreOffice 6.0 Basic, OLE) to illustrate the use of the API and demonstrate how to benefit from the included word processor, spreadsheet, presentation software, graphics program and database of LibreOffice 6.0.
Perhaps I can extend the database as well? I don't know enough yet to make the call, but this seems like the best place to start.
I'll need to look into licensing concerns, I know LibreOffice and use any advances in OpenOffice, but the inverse doesn't hold true. I'll need to make sure I can extend the software and ship as a standalone product.