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This edit suggestion/approval process would be useful for any entity, so I wouldn't bind it too hard to comments.
For the backend, when a user makes a suggestion, you'd make an ElggObject with subtype suggestted_edit and access ACCESS_PUBLIC. "Public" allows it to be seen by the original author, but it won't automatically be exposed in listings/search. Then make a "suggestion_for" relationship to the original content, in this case a comment object, and send a notification to original author.
You'll need to make a page that shows the comment and the suggested edit, ideally with some kind of fancy diff view. You'd have approve/disapprove buttons, but also maybe a comment stream so the two users could discuss changes and the proposer could make changes to the suggestion.
If you develop this as a plugin on GitHub it'll be easier for us to help.
I will take a stab at the basic stuff first and leave fancy/advanced features for later. I will put it on github once I have something working.