I have about 5 emailboxes in the New Outlook 365 version. I have added each Inbox to the Favorites pane, which already helps to quickly see where I have new emails.
However, it would be great if I could create a custom Favorites tab in which I can apply filters that allows me to see all unread/flagged/pinned emails from all my mailboxes. It gives an immediate overview of what things I need to answer, etc.
The same could be said about the To Do pane. Nice that you can select each mailbox/account individually, but an overall glance would be ideal.
Not sure if this is the right place to come up with suggestions, so happy to hear if I need to be elsewhere for this.