I think that dc subjects needs a rework, I don’t like the piece of body in the subject, (even some user complained about it, because “the message body is protected by the law(it is illegal to check it), but the subject isn’t”
Anyway I don’t think letting the user select the subject format would be good nor for users nor for developers 
if users needs to write an email instead of a chat message what they need is a email view in Delta Chat: [Discussion] Make DC a replacement for MUAs? - #12 by testbird