Combining multiple rptdesign / multiple datasources to produce one PDF

Hi,
I developed a web service architecture where the user can specify some filter criteria and then produce one out of 5+ reports. Either this is displayed in the browser or the user gets a PDF back. So far, so good.

Now I want to create some kind of batch process. So the user defines the filter criteria, chooses which reports to be generated and then in the backend I want to create ONE single PDF file containing all chosen reports. I think that I will not succeed to do it with subreports because 1) the data is dynamic and each report has a different dataset (which is provided in a Java List<String, List<String>> object which I pass into the single reports). And 2) from my understanding in subreports every possible subreport has to be specified, which doesn't let me add potential future reports without having to modify the whole subreport document, right?

So, well, I just don't really have an idea how to go with it and I am very thankful for every input which brings some light into the problem!
All the best, Jakob