hey belial, Uzumaki won't be making it to the next version because of legal reasons, players will be able to add their own custom elder gods with the in-game card editor though and I can't wait to see them! As of limiting mysteries, right now they are not connected in any way with the chosen elder god and you can easily treat them as Ito's short stories. The game focuses of mysteries, really. Elder gods are a way of adding more flavor, additional mechanics and god-specific events.
much love, Paweł