I'm sure Tom Sawyer island will be rethemed within 10 years. It's well-themed, but I feel like a gigantic portion of the people who visit Magic Kingdom never even check it out. Disney may want to move more people to the island if it gets too crowded. While Tom Sawyer is a classic book, I don't think today's children are that into it. I loved the book as a kid, but if I hadn't read the book Tom Sawyer's Island wouldn't resonate much with me.
Also, with Disney becoming more hypersensitive about anything that could be perceived as problematic (changes are being made to Splash Mountain and Jungle Cruise, while Peter Pan/Country Bears/It's A Small World and Enchanted Tiki Room are being reviewed), I think Tom Sawyer — which is based off a book that repeatedly uses the N-word and has a villain named "Injun Joe" may be eventually viewed as a liability for Disney.
So if Disney wants to retheme it, potential ideas are:
1. Neverland (themed around Peter Pan, though with all references to the Indians removed). You can have Captain Hook's ship, the Skull Cave, Pixie Hollow, the Lost Boys hideout, the mermaid lagoon. Possibly an E-ticket dark ride or simulator could be added to the land.
2. Villain Island — a land where Disney villains reside. The idea is they've been banished from the rest of the Magic Kingdom, but are currently plotting their revenge. (could maybe connect the island to an expansion pad north of the Magic Kingdom if it proves popular).
3. Sleepy Hollow — a Headless Horseman ride could go here.
4. Moana's island.
5. A New Orleans-themed land. This could help with the Princess and the Frog retheme of Splash Mountain (which is adjacent to Tom Sawyer Island). PatF Mountain is out of Place in Frontierland, but if they remove a lot of the Frontier elements around the new Splash Mountain, it could maybe work.
6. Halloweentown (The Nightmare Before Christmas)
7. Zootopia (would fit better here than Animal Kingdom)
8. Kumandra — A Raya and the Last Dragon-themed land. Could be visually stunning.
9. San Fransokyo (Big Hero 6)
10. A Pirates of the Caribbean island (if the existing attraction isn't enough)
11. Hawaii (themed after Lilo and Sitch)
12. A Star Wars land (preferably one with a forested area like Endor or Kasshyk)
13. Agrabah, with a restaurant and an E-ticket Aladdin ride
14. Syndrome's Island (from The Incredibles)
15. Narnia (very unlikely since Disney ditched the series after Prince Caspian and Netflix has the rights to make a TV show, but would be a cool walkthrough)
16. A Fantasia-inspired land filled with mythical creatures from the Pastoral Symphony and Nutcracker segments. Big Thunder Mountain (which is near Tom Sawer's island) can be rethemed to Bald Mountain to accommodate this change.
17. Land of the Dead (Coco). While I think Coco belongs in Epcot, that may not happen if there is no room to expand the Mexican Pavillion.
18. The Hundred-Acre Wood. A Winnie-the-Pooh-themed area.
19. Wonderland, with a maze and meet-and-greet characters.
20. Relocate Swiss Family Robinson treehouse to Tom Sawyer Island, and make it a Swiss Family Robinson Island. It wouldn't generate much foot traffic to Tom Sawyer's Island, but it would free up some space in Adventureland for a new attraction with the treehouse out of the way.
21. Wakanda (unlikely and would only legally be an option in Disneyland, unless Florida is somehow able to reach an agreement with Universal Studios to use Black Panther in the Magic Kingdom).
22. Arendelle. Frozen has tons of representation in the parks, but surprisingly little in Magic Kingdom. Disney may want to give Frozen its own land.