Excerpt from the thread -

Salesforce has invested heavily over the past few releases in the Case Feed, which leverages feed posts for commenting (in addition to Case Comments). This supports longer comments, rich text, @mentions and more in Lightning (live feed, code snippets etc).

With regards to Case Comments, there was an evaluation performed to enhance case comments to a standard salesforce object. This would allow you to add fields, customize the layout, run workflow etc. Unfortunately there are 2 main challenges to this.

1. Case comments are intended to be light weight, and presented in an easy to consume scrollable, format (eg - case feed, related list with focus on the truncated comment body). Having a rich comment object with custom fields does not easily fit into this

2. The effort to enhance/ replicate/migrate case comments to a new standard object is a significant scope of work, and needs to live in parallel to the existing comment framework

For these reasons we have encouraged customers that need more from comments to

- use the case feed as it allows more flexibility in adding case notes, comments, as well as filtering and presentation. See screenshot below which shows filtered case comments. Comments can be expanded and collapsed together in a single click or individually. Other case updates can be filtered as well.

- create a custom comment object. This would have the same fields as Case Comments, and any additional ones that are needed

If this is not an option for you, please do vote and post on the idea. I know it's been outstanding for a long time, but votes help significantly in prioritization of feature delivery.