One idea from me when I created the Notepad was to have support for PGP/MIME. At least for simple encrypted messages and only top level parsing like we do in GpgOL. So no subtrees. Basically it should show text from multipart/mixed, alternative, related and the attachments.
The use case I have in mind is that you have a webmailer or any MUA without PGP/MIME support. Then you get the pgp_encrypted_message.asc shown as an attachment. This attachment is linked to Kleopatra so you can open it with Kleopatra. Kleo will decrypt it but the user then has a mime structure that cannot be easily parsed / attachments extracted etc. Kleopatra could detect that and parse the message. We could use KDE Frameworks for that if we can avoid an akonadi dependency. Afaik the objecttreeparser is in messagelib and very closely related to KMail. So we can't really extract that.
I wonder if the Notepad would be the best place for this. As this will most likely occur after decrypting a file a standalone dialog might be better. The notepad mostly makes sense if we would want to also create MIME Messages but I do not think that this is really a big usecase.