Message list redesign
Following this issue with K9 mail: https://github.com/k9mail/k-9/issues/1859
I wonder if you'd agree having a slight redesign of the message list to be a bit more modern (like in the screenshots above) Typically, this would involve changing the font/color/size/position of the different message's items and work around the current useless badge on the left of messages
Also for the message viewing too, some design work is required to have a better looking experience with the software.
If you're ok, I can do some work on this and send a merge request when I'm happy with it.