I would choose Ursula; something about Underwater and Underworld
I hadn't thought of that one. You have to think about their elements though... fire and water. If they got together it would be steamy but it'd end quickly and if either one survived they'd be way worse off for it.
They seem like the type who would be better off keeping their distance and being mutual allies against forces that threaten them both.
I think Ursela would actually be a better match for Poseidon. They're both watery, and Poseidon had a bit of a temper at times. King Triton is actually a character from Greek Myth, he's supposed to be one of Poseidon's many sons.
The only problem that could occur with that is some older sources have Ursela and Triton being siblings. The Disney Princess line is apparently retconning this though and having her have some kind of unrequited love for Triton and them being unrelated...
I guess one way it could work if they were siblings is if they were uterine half-siblings (same mother, different father) since then Poseidon wouldn't be Ursela's father.
The musical, unlike the original plans for the movie and the guidebook, does mention that the father bequeathed her the shell though... so it does imply that Poseidon's their father. However... it is possible that Poseidon might have just adopted her...
The thing is though that Triton is supposed to be the older brother, meaning it's not like she's from a past marriage or anything. To be honest, I always saw Ursela as being an older sister type, she always seemed older than Triton despite him having a huge beard and stuff.
Triton's mother in mythology is named Amphitrite. Traditionally, Triton had both the trident AND the shell. Amphitrite was Poseidon's main lady and a goddess herself so I can imagine why he would give children of hers priority over those of his other consorts who were more like temporary flings. In fact, perhaps he'd do that even if it wasn't his kid, out of love for Amphitrite and wanting to take care of her offspring?
Pos and Amph possibly had 2 daughters (it seems the research is a bit conflicted regarding this) named Benthesikyme and Rhode. It might be that Ursela is meant to represent one or both of them in some shape. For some reason Rhode seems like a better match because she goes out and does more stuff, Bent just marries some dude in Ethiopia and raises one of Poseidon's kids that he has with a snow nymph named Chione that she abandons. Not something very Ursula-ish to do.
The problem there is that while Poseidon had kids with a lot of people, Amphitrite only had children with Poseidon that's mentioned in mythology, so if she had Ursula by someone else (meaning she is a third daughter separate from the other two, if they are actually considered his daughters, which isn't official) that it would have to be original material on part of Disney to create that.
It does mention that initially during their courtship, Amph fled from Pos to be with Atlas until his dolphins talked him up. 'Fled to' is probably a big jump away from creating Ursula though, heh. Plus for Triton to be older than her, this would have to happen afterwards, so maybe she flees back a second time or something. Poseidon wasn't a very nice guy, I could understand it.
I can't help but wonder if the dynamic between the two isn't unlike King Arthur and Morgan le Fay. She's also a half sibling who tries to steal the King's magic weapon that is his right to Kinghood. In some versions she's also manipulating Mordred, Arthur's son, to accomplish that goal, similar to how Ursula manipulates Ariel to accomplish that goal.
I don't know how much mythology should be referenced here though, because the animated series, while saying he is Poseidon's son, says he is Neptune's grandsone. In mythology his grandfather is Cronus/Uranus. Neptune is actually the roman name for Poseidon, lol. So his father is also his grandfather
I do think the mythology is kind of interesting if you consider the new movie coming out 'Ariel's Beginning' where her mother is Athena. In mythology, Triton had a daughter named Pallas by, and was a foster father to Athena. Then later in a fight Athena kind of offs Pallas. It's kind of hard imagining them getting married and having kids after that... Although if they changed it to just a 'protective older brother' (because actually, they're cousins, just one is much older) kind of role, I could see it working. For all we know the merpeople live for centuries after which the age difference could be pretty minor and they could forgive each other and stuff.
Athena also actually beat Poseidon in a contest for Athens, Greece, so it'd be funny if they incorporated this into the movie somehow. I don't know if this happens before or after she kills his granddaughter though.