Belle may have not been given too much to do yet, but she is the true love of someone who absolutely is a core character. Her connection to Rumple makes her important by default- far more so than Granny or Grumpy or Blue Fairy or Red. She isn’t just a side character- she is a big part of Rumple’s arc. Emilie de Ravin has stated that she is going to continue being a series regular in the 3rd season, and Adam has confimed on Twitter that they have plans for her. She isn’t going anywhere. We will see more of her story.
As for Ariel…I can see them doing the Neverland angle I suppose, but I am not too keen on the idea at the moment, especially if the Neverland mermaids curse sailors who fall into the water. I just don’t see Ariel being that way. I think it is more likely that she lived in Enchanted Forest. However, I have been wrong before. I am not sure about the idea that mermaids can travel through worlds either…that makes it sound too easy. The premises of the whole show is that it is difficult.