I sketched a quick plugin - Wikilinks - that adds basic wikilinks support to the Markdown editor (desktop only for now). This means that existing links to note titles / Joplin note IDs / Zettel IDs should work with Cmd/Ctrl + Click. There is no autocomplete of links yet, but you can use Joplin's native link completion (Alt+Shift+L, or Quick Links), and then hit Ctrl+Shift+L to convert any (internal) Markdown link to a wikilink.
UPDATE: Release v0.4.0 adds support for the Markdown preview pane, and mobile support (including the mobile editor).