Being able to mark an email in a to-do system as "Waiting for reply" with an expiration date would be useful. This way, such to-do system could alert you when

Being able to mark an email in a to-do system as "Waiting for reply" with an expiration date would be useful. This way, such to-do system could alert you when an email has not been answered to and prompt the next action. Emails could be cross-linkable and have tags (among the tags, there could be the ID of the project they belong to).