A PDF editor app would be very helpful. The best option would be an app that could edit PDF files like normal text files. But already having the possibility to split or merge several PDF files would be very helpful. Sometimes scanned documents are flipped over, so a rotation function would also be a good point to start with.
My wish would be highlighting pdfs, adding notes. NC could become not only a place for storing pdfs but also for reading and working with pdf documents, scientific papers, for organizing knowledge.
The thing you requested may be too complicated for a stand-alone thing like Nextcloud. Something like that to edit pdf comes to my mind. But here it would be too complicated and would freeze the platform as well. Simple annotating would be fine, some sort of what Adobe Reader has
This would be possible by leveraging LibreOffice Online (Collabora Office) when exposing Draw as LibreOffice can edit PDF’s using Draw. I’ve discussed this a while back with them and this is pretty doable, also adding notes, stamps, highligthing etc would be doable after that. Just a matter of having someone that is able to perform such task or a organization willing to pay to do the job.