They could be combining Triton and Poseidon, but I’m not sure why they would since Triton is more well known from TLM.
At any rate, I think Triton and Ursula are going to be siblings.
Since we’re likely getting flashbacks of a Young Ursula according to that casting call for 415 (congrats on calling it), which is also the same episode Poseidon is slated to appear in, it makes sense for it to be Poseidon.
Anyway, since 413 seems to be an ensemble centric, 414 is Maleficent centric, and 415 is Ursula centric, I’m going to guess 416 will be the Cruella centric.