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.
That would actually be kind of awesome, though no doubt this show would find some way to screw it up.
This would…actually be very interesting. Adam and Eddy were very cryptic in the Hot Seat when asked if Cora would ever be seen, “Can’t say for now”, usually means yes or not yet.
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"If you go as far as you can see...you will then see enough to go even further." - Finn Balor