See, I can’t see Ursula having any long-term use other than under the water. She’s a cephalopod in Disney’s version and unless she can transform herself into human I don’t see her being a big villain down the road. I’d be okay with Regina taking over the role.
I think Ursula’s role largely depends on how much they want to use Ariel and where in Ariel’s story we are when we met her. If she has never become human then Ursula will probably play a larger role and not be Regina; if, like Macy suggested a few pages back, the events of the Little Mermaid as we know them have passed and Ariel is older than her normal 16 years,then Ursula will probably only be used in one or two flashbacks when we learn about Ariel’s past and in that case, she could be Regina. If the big bad of S3 (part 1) is PP/Tink/THO and the big bad of S3 (part 2) is THO in SB and S4 is mostly Oz (wild prediction based on only a few hints) then Ursula doesn’t need to be any more than Ariel’s personal enemy and not the enemy of the whole core cast.