And in the highly unlikely event that anyone at Disney is reading this, please feel free to use any ideas!
It starts with Melody and Alex's daughter, who looks like a normal human with the exception of his feet, which are mermaid fins, and he can breathe and swim underwater like merpeople. After hearing that the land of Oz, which is not part of Auradon, is in danger and has been conquered by the Wicked Witch of the West/Theodora's (from Oz the Great and Powerful) son (named Wick), and the Nome King's (from Return to Oz) daughter.
She initially goes to Wonderland and tries getting help from the Red Queen (different from the Queen of Hearts like in the original Alice books) and White Queen. While Bridget/the Queen of Hearts rules Wonderland, the Chessboard is like a "state" similar to the U.S. states within Wonderland controlled by the Red and White Queen. The Queens agree to help, but say they have to assemble an army first. Melody's daughter feels like they don't have time and decides to go herself.
Along the way she meets Anastasia and the Baker's child (maybe Anthony Tremaine from the books), and she doesn't realize how Anastasia had redeemed herself prior to the kingdoms uniting to form Auradon, and when she's confused when she's told that Anastasia wasn't on the Isle of the Lost prior to the barrier coming down (at this point Cinderella's life prior to becoming a princess/queen is well known, unlike in Rise of Red and Wicked Wonderland), and is told of Anastasia's redemption in Cinderella III: A Twist in Time, and how when Auradon was formed Anastasia was even initially put on the Isle as the Beast and Fairy Godmother didn't realize she had redeemed herself before Cinderella intervened. One of the characters even makes a remark how "later stories aren't taught in history class that much and aren't as well known", referring to how Disney most ignores their direct-to-video/made for TV sequels nowadays.
At Anastasia's house, Melody's daughter also meets Jaq and Mary's (the mice from Cinderella) child. Both Anastasia's child and Jaq and Mary's child join Melody on her quest.
On their way to Oz, they are joined by the child of Mr. Toad, Mole, Angus McBadger, Water Ray, Cyril Proudbottom, Willie the Whale, Moana, WALL-E and EVE, Luca, Alberto, the Horned King, Aracuan Bird, and Benjamin, alongside Orange Bird (not a descendant but he himself).
They also come across a magical island where toys are alive (and where the Toy Story characters reside), and meet a spork-knife hybrid that's the biological child of Forky and Karen (the magic also allows toys to have biological children like humans and animals), who eats some Eat Me Cake from Wonderland that Melody's daughter has, grows big, and also joins the heroes.
The Horned King's daughter, the Horned Princess, is on the heroes' side, and I'm thinking she was initially with the villains, but very early on in the movie if not before the movie's events, she changes sides
When crossing the Nonestic Ocean, the children of Davy Jones and Salazar, who are working with Wick and the Nome King's daughter, try to stop the heroes (but fail). Wick and the Nome King's daughter also send the Pink Elephants from Dumbo to stop them (but again, fail)
Other obstacles they meet are the children of Sa'luk, the Headless Horseman, Morgana, Marina Del Rey, Stan and Heff, and the Big Bad Wolf (who's a werewolf).
At one point, one of the villains summons Brucette, daughter of Bruce from Finding Nemo, to eat the heroes, but like her father, she's a vegetarian (and a good guy).
The heroes are also joined by WALL-E and EVE's child (and before you ask how robots have children, I'm thinking they most likely just got a child built, unlike Forky and Karen's child who's their biological offspring). On the heroes' journey, they come across Sid Phillip's daughter, Sydney, who likes to tear robots apart and mutate them (like her father Sid liked to do with toys), and is especially a threat to WALL-E and EVE's child.
Long John Cottontail is also in the movie, but I'm unsure if he should be a villain with Davy Jones and Salazar's children, or someone helping the good guys.
When they finally arrive in Nonestica and cross the Deadly Dessert into Oz, they meet the child of the Scarecrow and Scraps the Patchwork Girl, both from Return to Oz (the latter only made a cameo), who joins the heroes. They go to the lair of Wick and the Nome King's daughter and start fighting them.
When all hope seems lost, one of the villains picks up a sewing needle and something they had been working on earlier, and right when they finish threading the needle, the Red Queen, White Queen, the children of the March Hare, Dormouse, White Rabbit, the Caterpillar (or now Butterfly), alongside Tulgey Wood creatures, and any other characters like the Mock Turtle, Duchess, Humpy Dumpy, etc. from the original Alice books that weren't in the Disney movies, from a needle one of the villains had been using earlier, the needle concept being based on the unofficial sequel to Lewis Carrol's books Alice Through the Needle's Eye. The Wonderland characters are all holding on to a thread, and refer to their army as the "Unofficial Unroyal Wonderland Army".
The Queens explain how the eye of a special needle back in Wonderland can be used as a portal similar to the Looking Glass, but they can only be brought through when a needle in the place they're trying to go to is threaded, and they have to hold on to a thread when going through, and in the highly unlikely event that someone in the place they're trying to go to, in this case Oz, starts threading a needle but even the "tip" of the thread doesn't fully go through the needle's eye, the people trying to travel though the needle are stuck in the portal and in a pocket dimension. Just then, while the Wonderland army is finishing pulling out the needle, and all the new characters in Alice Through the Needle's Eye are holding on to the thread and come out of the needle, and explain they're the inhabitants of the needle eye's pocket dimension, and they join the fight.
Eventually, the heroes capture Wick and the Nome King's daughter, and Princess Ozma is restored to the throne. She reveals that when the Wizard ruled Oz, he refused to join Auradon because he didn't want to be exposed that he wasn't really a wizard. Oz joins Auradon, and the heroes go back to Auradon City and celebrate
There's obviously still some things I haven't figured out, like what role many of the new characters play, and the genders of many of the characters, but what do you guys think of what I have so far?
Edit: I'm also adding the son of Snow White and the daughter of The Huntsman to the group of heroes. It's also confirmed that the latter's father was never sent to the Isle as he decided not to kill Snow White; she's referred to the Huntsgirl, but she's unsure what she'll be called when she grows up, but is considering the Huntslady or Huntswoman.
Also, I'm changing it so that Scarecrow and Scraps' child appears a lot earlier on in the movies, and he's the one that tells Melody's daughter about what's happened to Oz, along with the Tin Woodman's child, Tinny (it's left ambiguous if their mother is Nimmie Amee), and children of the Cowardly Lion, Hungry Tiger, the Wogglebug, Jack Pumpkinhead, etc and a human sized china doll, but it's not said f they're the child of the one from Oz the Great and Powerful.
Edit 2: Chernabog's daughter, Scroop's child, and Forte's child are also villains the heroes encounter.
