It would be wonderful if we could ditch "change these lines" type comments in favour of just letting merge requests with the changes be easily surfaced.
I was at one point considering writing a tool which would checkout an MR, then let you just edit it as per normal, and then would submit the whole thing back to a Gitlab MR as a set of proposed changes. The point was to ensure that you could easily expand the MR beyond the diff of changed lines, which was frequently inadequate to review a patch properly since it omitted context.
The UI doesn't know what to do with it, and most developers don't really understand it without spending a bunch of time explaining it.
When it works, it works great but the overhead is ridiculous since you get nothing inline on the UI: ideally something like the commits should pop up as comments, with accept/reject/discuss options.