KMail already has a pretty good preview for if the attachment is a mail itself (message/rfc822).

What I would like is, an option (right click/drag 'n drop) to
1) Respond to the mail attachement. This already works with detour (View Attchement -> Respond)
2) Save the mail somewhere in the mail folders. (You can only save them in the file system.)

1) The first option would be nice if you are the moderator of a mailman list for example. (For mailman it would also be nice to better support more headers while writing mails, like the Approved-Header.)

2) This is useful if you get a false-positiv spam mail. Some spam filters send the original mail as an attachment. Would be nice to just drag and drop the mail to the right folder in KMail.