Yeah, it doesn't seem like it would be hard to do. One could easily make an Ariel by putting Poison Ivy's hair on one of the POTC mermaids and Eric by putting the new Robin hair on Han Solo from Jabba's Palace, and we already got a yellow king Triton, so they simply would have to make him fleshy. The only new molds they'd have to do are Sebastian, Flounder, and Scuttle.
However, one has to consider that if there was a System Disney princesses line, it could very well have the Friends-style minidolls considering how well Friends is doing and that this line would be aimed primarily at girls.
I'm still holding out hope for Monsters Inc/University sets, but I guess with all the sets coming out in the summer already, including the Lone Ranger sets, the chances are slim, and my wallet will be thankful if there wont be any.
By the way, does anyone else find it strange that the Undersea Castle has a lower set number than the Boat Ride? I'm guessing the Boat Ride is based on the "Kiss the Girl" scene, which would be a fairly small set, so I can't imagine the castle to be an even smaller set unless it's really just Ariel's room like the Sleeping Beauty set, but then why would they call it a castle?