Visual UML diagram editor
There are plenty of cases when diagrams should be attached to issues\wiki pages. Right now I'm creating a diagrams in external tools, and then exporting them as png file + xml, attaching them to issue to have an ability to collaborate.
Integrate visual diagram editor. https://github.com/jgraph/draw.io and have an editor right in the GitLab. There is no need to add new markdown or learn new syntax, it's awesome frontend WYSYWIG editor.