Unfortunately, based on K & H’s comments, it looks like we either won’t get Ariel anytime soon (end of the season, if at all in S3) or her story will be some short one-and-done type deal, which no one wants. However, it’s a very really possibility because in the original, the mermaid and the prince don’t end up together; she saves him from drowning but he thinks another girl did it and falls in love with her and the mermaid does because the deal sheave with the sea hag was that if she didn’t get him to fall in love with her, she would die. So I could see K & H using her to save someone she loved from afar: Peter Pan, possibly, since it sounds like in Neverland both he and the mermaids aren’t very nice, or Hook, or maybe even Phillip from the Wraith.
Or K&H could wait and make her story awesome and Ariel a major character.
Also I predict that Ursula won’t be a new character, but either Regina “mermaid trouble” line) or Rumple (“squid ink” line and penchant for making deals).