The BookStack has an impressive use of draw.io in their wysiwyg editor. There is often use for drawings to show what you mean in a written topic. Please try their demo (bookstack) to be inspired!
History / Revert to previous version
Add the ability to view the changes history of a page and the possibility to revert to a specific version.
Dropdown Navigation Items
Support for child navigation items to allow dropdowns in the main navigation pane. If you have a large wiki, it can be hard to organize your pages without some type of collapsible navigation.
Sidebar Navigation Permissions
Users should be able to permission the sidebar. Maybe we can associate a set of groups with a nav item, similarly to how we can assign users to a group when managing groups.
Switching from an editor to another on an existing page
As a page editor, I want to be able to select another editor to edit a page, so that I can use my favorite editor on any page.
Breadcrumb for subpages
Should (auto) generate breadcrumb links on all the subpages based on the hierarchy of the file.
Reduce pageHistory table footprint
When a page is modified, the full text of the page is saved to the pageHistory table in the content column. This can eat up both storage and memory quite fast for large pages with many changes, especially in multi-user wikis. I would suggest storing only the changeset (i.e., the differences only, probably as a BLOB) instead of the full text. For Git-backed wikis, I would suggest using Git for page histories instead of this internal system. I would also suggest an option to disable page histories altogether, as well as an option to permanently clear the pageHistory table.