In regards to not having any villains for Season 6 (if the Kings of Darkness are used for 5B), there is an easy way for them to fix that.
IF 5B is the Underworld, all of the past villians will be there. The 5B cliffhanger could be them all escaping and coming back. And then Season 6 can be the final battle that Rumple predicted in the pilot.
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