This is probably me expecting too much out of the writers (as per usual), but I think (hope) that the cast connection Hades has could potentially be a combination of what a few have suggested. I’m thinking Hades could have been a fourth party in a missing piece of the Cora/Rumple/Regina story, though this mainly hinges on the baseless assumption that they always planned on bringing Barbara Hershey back in the Underworld, seeing as how Cora/Kore is another name for Persephone.
This is a good theory….it’d definetly explain how Cora got the position as Mayor of the Underworld if her and Hades were already close friends/perhaps lovers. Boy, Cora sure did get around didn’t she.
"If you go as far as you can see...you will then see enough to go even further." - Finn Balor