I'd really like to see a return to Atlantica for a run through The Little Mermaid II: Return to the Sea. I always forget Ariel wasn't one of the Original Princesses of Heart (which is weird since she is an actual princess, unlike Alice, but that's all cool.) I think it could be a fun little plot point to explore them returning to Ariel's world, and thinking that she's one of the new Princesses, only for it to turn out to be her daughter Melody. It would also nicely tie in to the storyline for that movie (Melody: "Mom, I'm not a little girl anymore, you can't keep me locked away forever. Look, I'm one of these magical princesses that needs to save the world!" Triton: "I guess I have to be open to the idea of traveling between worlds now that it's happened to one of my family members, but at least it provides me a good excuse to finally see my granddaughter.") Plus, I could see Ursula's sister being a fun boss battle to play through.
I wouldn't mind having Kiara from Lion King 2 be one of the princesses as well, as I'll take any excuse for another Lion King world, but I'm super skeptical they would go with a non-human princess.
Someone once mentioned to me that they thought the Princess and the Frog would make a really good KH world and that the main villain would be an awesome boss battle, and I concur, so it would be nice if we could have Tiana as one of the new princesses.
I also wouldn't mind seeing a Wreck-it Ralph world, and I think Brave could be a good addition (though it might be a little too similar to Rapunzel.
So we've got:
1. Elsa/Anna (I don't 100% like how they made them both one, so maybe we could say they share their power?)
2. Rapunzel (If the above doesn't work maybe they'd say she lost her bower when she cut her hair, but that would be kind of silly.)
3. Kairi (if she still is one. I know she is and probably will stay that way but they could make up an excuse about her power being passed on due to the ending of KH3, though it's highly unlikely.)
7. Merida (Brave)
Bonus Round: Kiara (Lion King 2) and/or Eilonwy (Black Cauldron)
I doubt the Black Cauldron would ever make it in, despite it's cult classic status, but you never know. I also wouldn't mind seeing Princess Lea if they ever incorporate Star Wars but that would be super touchy in light of Carrie Fisher's passing. (R.I.P.) I could see Disney not being super concerned by it, but I could also see them being nervous about the backlash, and I know the Square team might not be super into the idea since it's a custom in Japan not to replace voice actors after their passing (the reason why Phil had no speaking lines in KH3, due to his Japanese VA passing before recording could be completed.)
I like the idea of adding in Atlantis too, so I wouldn't be opposed to that. I would prefer not making Boo one, but do admit it's a possibility. She's just so young to place in a dramatic situation without having to have it play for laughs or be goofy, and you would think Vanitas would have mentioned that during the Monstropolis level if she was one of the light. (unless he did and I'm forgetting?)
EDIT: ^^^ Moana would also be cool, and Namine would be awesome (fan favorite here lol)