Hi,
I think it would be great if we could add a unified "all mail" folder which would by default show all inbox and sub-folders emails -- so all emails, except sent/outgoing/template/draft.
I find this a rather convenient tool in a dedicated mobile app that I use, especially when the number of accounts and sub-folders grows and you are looking for an email without clearly remembering where it landed.
And just very useful overall to get an overview of recent messages!