Right now, the change from "add a comment" to "suggest improvement" is an experiment on Interpersonal Skills. If the experiment goes well there, it may be expanded to other sites and perhaps the whole network. Also, many other features and functions of the network are being looked at for improvement in order to better explain how to use them or to give new capabilities to help onboard new users or aid power users in doing the things they want to do as part of making the sites on the network more welcoming.
I'm not opposed to turning it on, and if there's strong interest, I can even ask if SESE can become part of this experiment. But I don't see a huge need for it - many comments left are constructive questions about the content of the post they are attached to. The one improvement that I can see is that contents of any back-and-forth in comments can be brought up into the post via edits, but even that appears to happen regularly (if I'm mistaken on that, please let me know).