@King Kindred I don't like the idea of Terk being a princess. Then again, i've always been a fan of the idea that "Princess Heart" is just a special heart that appears at least once in every world. It's just some are much easier to find than others and there would be little clues here and there. So, Princess & The Frog world, Tiana would have a Princess Heart, but this wouldn't become obvious until after she married Naveen.
Ultimately, they only need 7, but bad guys are going after the Princess' that they know of already. Aurora would also be a Princess, but Maleficent prefers to leave her in this perpetual state for her own amusement and may drag her out if push comes to shove, but right now she can have her cake and eat it, so no need to upset that.
That being said, if we are going with "Only 7" My recommendations are:
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Star Butterfly (Would like to go to Star Vs world at some point)
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Megura (In the original mythology, her father was the King of Thebes, and if the endgame takes place on her world, it makes sense)
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Mira Nova (Ties in with Launchpad and Leon's Star Command backstory)
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Rapunzel (Recently rewatched the Tangled series, it's pretty good, i recommend it.)
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Dotty (She would be descended from Cadpig, who, in the original 101 Dalmatians book sequel "The Starlight Barking" was the Prime Minister of all dogs, close enough to royal blood.)
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Cadpig (For the above stated reason)
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Princess Shayla (Tying into Zen-Aku's backstory of him trying to find her, despite having no idea who she is.)
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Dolly Dalmatian (Her major love interest in the series is Prince Corgi, Plenty of other Princess' only got there via marriage.)
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Princess Garnet (Being that i've tied Karre's backstory into FFIX, it would be an interesting dynamic)
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Webbigail Vanderquack (Either version, although thanks to that stupid fucking twist at the end of the remake, i suppose remake Webby makes more sense.)
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Fox Xanatos (Her Mother is Queen of the Fae)
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Kim Possible (If Bonnie hadn't have cheated, she would have become "Homecoming Queen" And that's close enough for the powers that be.)
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Princess Eilonwy (No specific reason, i just like her as a character)
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Kiara (Again, no specific reason, i just like her)
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Mirabel Madrigal (The reason The Miracle can't grant her power is she already has too much. Speaking of, i kinda like the idea that The Miracle itself is a disembodied Royal Authority, which is how it grants powers. The Authority inhabits the candle, but the being the Royal Authority comes from now inhabits Casita. I'd let you decide on who it used to be, but it's an idea.)
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Fa Mulan (Another "No specific reason, i just like her" But she is actually an official member of the Disney Princess range...
Also i may have a mildly unhealthy infatuation with Ming Na Wen)
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Princess Leia (Yet another "No reason, i just like her" Although i still say that Star Wars and Marvel should play a role more like Final Fantasy within Kingdom Hearts, in the sense of "These characters don't have a specific world, but they are used to fill in logical roles in other worlds" like how Zack was from Hercules World and Setzer and Vivi are from Traverse Town.)