My idea is based on what Docmosis could do, except the template is a md document. As a input I have a java model, and the output would be a PDF file. Do you think it would be possible to do that with flexmark-java?
I am not familiar with Docmosis but don't see why markdown generated from a template would make any difference to the parser or the renderers.
Once you have a Markdown source, no matter how you derived it, you can parse it to an AST and then render the AST as HTML, PDF or DOCX or any combination of thereof. Also plain text rendering is easy to implement and I can easily add it. Although some users have already done that.
The parsing and rendering part is easy and is part of the library. The templating part is to be implemented by your code and I see it as being mostly independent from the library. It generates Markdown then uses the library for parsing and rendering it by any supported renderer.
If there is a need to add more renderers it can be done. If there is a Java library available that manipulates that format then the task is fairly easy. I implemented DOCX rendering in a week which included learning docx4j and intricacies of the docx format.
I think it is an interesting project. Let me know if you need more input from me.