I could also see Pocahontas or Rapunzel. I picked Snow because she's gullible at times and would do anything to be with her prince which would lead to...

Ursula would need a princess she could manipulate by offering her her dream. I could see Rapunzel as that princess because she was willing to go with Flynn even though she had been told that the outside world was full of horrible people, and even Flynn said he wasn't worthy of trust, and she did it because it would lead her to her dream.

^ this and also Aurora because I can see her feeling exiled like Maleficent did

i agree with rhythmicmagic and the 'something in return' that Ursula would want would be Rapunzel's hair... lord knows, Ursula needs it!!

Her father is a powerful sultan. Ursula only uses Ariel to get Triton's power, so she'd probably do something similar to Jasmine to get the Sultan's ruling power

^ I agree with that. It's power, a political power that Ursula longes for, because she already posesses an immense amount of magical powers therefore I cannot see why would she even need/want Rapunzel's magic hair. She is good at manipulating, yes, but the only person who ever managed to be able to trick or manipulate Rapunzel was Gothel because she brainwashed her to the point where Rapunzel trusted nobody else BUT her. She didn't trust Flynn in the beginning that much and even if it was naive/delusional of her to assume, she sincerely believed she had a situation under control and that Flynn is actually scared of her frying pan and ability to defend herself. I can't see Rapunzel trusting Ursula, in fact, I can see her being scared to death of even the simplest contact with someone like her whereas when it comes to Jasmine, like Ariel, she could be aware of Ursula's evil ways and not naive about the possible consequences of any deals with the sea witch, however, if it meant achieving something she longed for I can see her making a deal with Ursula, whereas Ursula would try to use her to get to Sultan and his power.

I agree with Mongoose09, plus she's easily convinced and really gullible.

What the others said...