DC uniqueness - it is simplify email messaging AND automatic encryption.
Attempt to seat on two different chairs, located in opposite corners of room, that what this tread all about IMO.
KISS principle - do one thing, but do it best.
If one need full feature email messaging, then he/she should use full feature email client such as thunderbird for example that allows feature one needed.
Personally I really don’t like that DC attempting to act as a normal email client for un-encrypted messaging. It defeat the whole idea of simplified messaging, idea to be a secure chat application. It may create situation when encryption will be broken logically by simply messing encrypted and un-encrypted messages.
Chat != email
Use the right tool for a job - is an old wisdom verified by time.
There are plenty of email clients that satisfy requirements to supply custom subject as well supporting encryption. Why one more ?
Just download a normal email client and use it.
Blue Mail, Email by Edison, Microsoft Outlook, Gmail, Aqua Mail, Email TypeApp, K-9 Mail, myMail, Newton Mail, ProtonMail… and so on, just choose your favorite tool instead of breaking unique features of DC and move it to the list of highly competitive apps listed above.
Adding geek’s “-” or “|” separators easily can be abused by people who have no clue about such proposed feature (I think it is majority of users who don’t like to read documentation) and DC will easily became non encrypted chat app.
While DC is technically a MUA, the gist of this app, that it is a chat app.
There no such thing as a subject in chat applications.
One more time: Use the right tool for a job - is an old wisdom verified by time.