I do not expect Brave to create this, but I think it would be cool to have a Brave Sync Firefox extension which when installed on Firefox desktop or Firefox Android, will disable Firefox Sync and instead you will use Brave Sync to sync your bookmarks, history, settings, password, etc.
This can allow users to have their data synced on both a Chromium based browser (Brave) and a Gecko based browsers such as Firefox or Waterfox.
Current Brave Sync mechanism uses a particular feature build inside chromium engine. It cannot be replicated on non-chromium browsers.
I also had somewhat a same point. All browsers, be it chromium or non-chromium should come together and form a sync feature compatible with all of them. Meaning you can sync stuff from chrome to firefox to brave etc.
I don’t think it will be possible as chrome, edge harvest sync data but private browsers like brave, firefox, vivaldi should have cross-syncing feature between them.
Unlikely it will happen.