(If looking for an K-9 email alternative, there is now also FairEmail on f-droid.)
I think the full standard email features may fit in with an eventual “Email” tab Add setting to show all people in DeltaChat (current contact request).
Only simple subject editing would make sense also within the chat interface. Some ideas: https://github.com/deltachat/deltachat-core/issues/128#issuecomment-423516046
There is even a bounty for editable subjects https://www.bountysource.com/issues/56768801-editable-subject-line.