Yes, and I think the idea of “chats” is very good. The default would not, nor should, have to change.
(The Email-chat spec simply allows to correspond with any email address, thus also those using classic email subjects.)
Deltachat for itself would not impose a requirement to use subjects in its messages. But if some contact starts writing with different subjects (to stay on top of a larger bulk of messages) deltachat could just show the subject at the top of the chat (idea from pcrockett), and provide a filtering possibility (a search box, with the list of subjects already used in the chat, similar to “suggested search terms”), that gets shown by tapping on the title, whicht could have some “drop down” or “expand” icon to indicate this feature.
Showing the subject in the title also reduces the redundancy that would result from displaying subjects within every message.
As long as no customized subjects are used in a chat, everything could stay the same as it currently is. (Except the small indicator?)
One remaining question might be how to best allow deltachat users themself to do a first custom subject change. A tap on the title currently opens tabs for the group settings, gallery and documents, maybe that could just as well open with a new (to be introduced) tab for “subjects” or chat “topics”, containing a “plus” button for adding a custom subject.