Please add wikilink syntax support. This is a simple and effective and inter-compatible way to reference documents internally to the markdown structure. This is, as is noted by other commenters, adopted by competitor Obsidian, as well as flatnotes (a web self-hosted option). This syntax is too convenient to be ignored, and already has traction.
It is also preferable to using import processes. This enables users to simply open their Obsidian notes vault in Joplin and work without thinking about how to migrate their vault in a batch process.
[[Please add wikilinks!!]]