Review "Suggest changes" - in GitLab 11.6
As a reviewer, you’re able to suggest code changes with a simple Markdown syntax in Merge Request Diff threads. Then, the Merge Request author (or other users with appropriate permission) is able to apply these suggestions with a click, which will generate a commit in the Merge Request authored by the user that applied them.
Just to track this feature for heptapod (I know there's quite a road to 11.6).
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information