Currently only the (-) list item syntax is supported for checkboxes. I think adding the two additional syntax options would be helpful because it would be consistent with GFM's task list extensions (GitHub Flavored Markdown Spec). This would make it less surprising for users coming from other tools that support this feature (github, vs-code, etc).
I also have a lot of old checklists using the (*) syntax that I copied over and it would be great if they just worked in joplin.