Comments in WordPress were originally thought of as a way for visitors to a site to have discussions on the site. I’m going to refer to the traditional comment idea as ‘local comments‘. This is where someone fills out a comment form on a post, which adds a comment to the page, either with or without moderation.
Also built-in to WordPress are pingbacks and trackbacks. These are where another site notifies of a link to the page and this is displayed as a comment on the page. Pingbacks and trackbacks have their issues, and admittedly, WordPress, except for some minor tweaks, has left the presentation and features of these mostly unchanged, save for some security efforts. Therefore, it has become common to disable them. Webmentions are a replacement for the older pingback and trackback protocols.
There are also custom comment types used for things like logs, receipts(See mention in Trac Ticket which notes that popular plugin Easy Digital Downloads does this), etc. which are not part of the general vision of what a comment is. Of course, custom post types aren’t all traditional posts either.
Meanwhile, other areas of WordPress are getting functions to register functionality. 1n December of 2015, a ticket was opened for Custom Comment Types. This would, in theory, mirror the functionality of Custom Post Types, where a registration function would declare the functionality and behavior of a comment type.
But there are issues here beyond what is identified in the ticket. How will a theme know how to display the new comment type? There needs to be a way for this to be specified by the registration and overridden by themes that know what that comment type is. There is a possibility there may be some breaking changes in that design.
Meanwhile, comments cannot be referenced uniquely and distinctly. There is another ticket equally old, proposing comment permalinks. Whether this is just to find the comment on whatever page it is on, or allowing a comment template that emphasizes the comment by itself, it is an aspect of the comment system that needs replacing.
That addresses the structure, but not what could be done with it. Not only would this allow for webmentions as a medium to create comments(webmentions being the successor to pingbacks), but support for responses that are popularly used today such as like, emoji, annotation, etc. The Semantic Linkbacks plugin tries to create these relationships in comment meta, but it is not as robust as a supported implementation would be.
Annotations and marginalia recently have been of interest in allowing people not to just comment on a post as a whole, but on specific areas of it. This could also be a promising piece of a comment overhaul…supporting this. In my contributions to the Webmentions plugin, I started this by supporting URL fragments(see fragmentions), which would allow for a specific part of the post to be highlighted.
Mentions