I hope I'm not being offbeat by self promoting but I currently am working on v2 of Unreplied, a Mac app I released two years ago that uses a status bar menu to show you iMessages/SMS from people you haven't replied to yet (including messages you've already read). The elevator pitch is imagine someone asking you if you want to hang out sometime. Depending on how busy you are at the moment you may not have time to check your calendar or reply with a confident answer, so you push it off till later. Inevitably you forget entirely and end up accidentally ghosting whoever asked you.
Slack and some email clients seem to do a good job of avoiding this by offering "remind me in X..." options, but those are solutions that users actively have to seek. Unreplied works passively in the background so there's no input required to generate the list.
Obviously there's no iMessage on Windows, but the concept behind the app remains unexplored on all platforms IMO. I think it would be very useful if all chat clients implemented some sort of "left on read" filter.