In a busy mailboxes, a lot of emails, are only for information purposes (CC, BCC, ...) and many replies don't need special response or attention. But some threads are important or some sended emails need responses, that will come hidden between big flow of emails.
I proposal a feature, that allow to mark in compose message page, [waiting reply], and when a reply of this email income, mark as separate colour, or mark with a flag.
The goal is improve the accurate on working. Actually the flags limitate they utility, because only mark the emails received or sended, and this feature wants to automarks incomming replies.
Maybe a final question character (?) in compose page it's enough to activate this function.
I think this is very usefull, and maybe was add as a plugin... is in thands of the community.