Migrating from Obsidian-Tasks has some helpful space scripts around tasks to handle them more similar to Obsidian Tasks. It might give you an idea of other attributes to support.
What I’d really love is a way to view and edit/complete tasks in another client. I don’t have a good suggestion though because even clients that support caldav VTODOs are usually not great imo. And there isn’t really another “standard” todo/task api to add support for.
When I originally saw you mention caldav, I was thinking it’d be cool to have silverbullet act as a caldav client and sync to/from something like nextcloud. (Instead of itself being a caldav server)