I often miss people responding to my comments because there's no way for me to know when someone responds, or posts a comment on the same question or answer I have.
My suggestion would be having the ability to be notified when someone posts a comment on a question or answer you've already commented on. This could be toggled, and maybe it's off by default. If not that, maybe the ability to 'watch' a post for any new updates, including comments.
My other request would be to have the ability to mention people in comments. This way, if you want to directly respond to someone's comments you could. This could also be toggled in settings, if desired.
Thanks for reading!