Not really a feature proposol - just something for the watchlist.
I just stumbled upon Mailtime.
This seems to be quite similar to Delta Chat - but with a different focus.
If i get it right its about: show all your mails as chat messages.
Which is not really something i would want as i use mail not for chats - where chat to me means: talking about stuff that is not really that important, does not include important info and for that reason conversations/quick talks like that can be deleted after some time (so basically a - guess what - chat).
But anyway - if you haven’t heard of it already - maybe you would like a look.