Would there be a way to modify the email envelope to cue the source of new incoming mail? Maybe this is only needed to be done via a theme or scrim fix?

It would be helpful in being able to see that in the notification bar, and maybe even a different outline color to id between incoming messages when you open the all inboxes folder.

Is this possible even for 2.x or 3.x or is this out of reach for access to tweak?

This seems like you could add it to advanced system framework and have a simple menu option to select a color per account?