Wow guys, thanks for responding so much! :D Your theories are great! And someone here mentioned Jafar in the Underworld... I have two theories on that:
a) Well I'm not sure that he could be in Underworld considering that he is a genie. But if he was in the Underworld it would be a great idea if they released him from the Underworld at the end of 5B and then he would be the main villain for 6A! And then in 6A the characters would have to go to Agrabah to search for ways to destroy him.
b) Maybe some of the characters (or just one, it doesn't matter) will end um in the Underworld and there, the main villain will be Hades. But in Storybrooke (or EF or wherever the rest of the characters are), they unleash Jafar and at the end of 5B they send him to the Underworld in the exchange for the person who already is in the Underworld. That way, they could do both an Agrabah arc and Hercules/Underworld/Greek arc, just like they are doing both Brave and Camelot arc in 5A.
I seriously love both theories and it would be amazing if something like this happened.
PS: I just got another great idea! What if Jafar or Hades was actually a dragon and one of them is Lily's father? Considering that we won't really see Maleficent and Lily until 5B, this would be an awsome twist. Plus it really agrees with the nature of the show... Everyone's somehow related. xD