Another option could be characters from Sofia the First. Sofia could visit Shimmer and Shine's magical world. Since Sofia has her own amulet with magic, she could learn from Shimmer and Shine how to use genie - like magic, and they could all go on an adventure to find a lost magical artifact.
Well, they could cross over with characters from other Nickelodeon shows like the Winx Club. The fairies from Winx Club could interact with Shimmer and Shine, sharing different kinds of magic. They could have a magical battle together against a common enemy.
They might also cross over with characters from Miraculous: Tales of Ladybug & Cat Noir. Ladybug and Cat Noir could end up in Shimmer and Shine's world. There could be some confusion at first as they try to understand each other's powers. But then they realize that they need to work together to stop a villain who is causing chaos in both their worlds.
A really fun plot could be a crossover with a modern - day human world. Shimmer and Shine accidentally end up in a bustling city. They have to hide their magical identities while also trying to find a way back home. Along the way, they befriend a kind - hearted child who helps them understand the strange new world, and they in turn use their magic to help the child with some personal problems.
Niki is another underrated character. If there was a crossover with a fairy - tale world, she could be a powerful fairy or a kind - hearted witch. Her baking skills could be turned into magical potions or enchanted treats. She has a lot of depth to her character that could be explored in a different setting.
Achilles could be an interesting character to include. He was a great warrior in Greek mythology. A crossover could show Spartacus and Achilles having a friendly competition in combat skills at first, but then they realize they need to work together to fight a greater threat, like an army of demons or something.
Naruto could be included. Since he has a connection to the tailed beasts in his own universe, it would be interesting to see how he interacts with the three - tailed beast in a different setting. Maybe he could help the three - tailed beast understand the new world it has crossed over into.
Characters from the 'Smallville' universe could be great. Clark Kent, for instance. Imagine The Flash and Clark Kent working together. Clark's super strength, invulnerability, and other powers combined with The Flash's speed would be an unbeatable combination. They could face off against a powerful Kryptonian villain who has come to Earth to cause chaos.
Oliver Queen from Arrow would be a great addition. His archery skills and combat experience could complement The Flash's super - speed. Also, Kara Danvers from Supergirl could be included. Her super - strength and flight would add a new dimension to the crossover. And of course, the Legends of Tomorrow crew like Sara Lance, who has her own set of combat and leadership skills.
If it's a crossover with the Final Fantasy series, characters like Cloud Strife could be included. Cloud's brooding nature and combat skills would contrast with Professor Layton's calm and cerebral approach. Tifa Lockhart could also be part of it, providing her own brand of strength and determination.
How about including characters from Sailor Moon? It would be really interesting to see the Gokaigers interact with the Sailor Scouts. Maybe they could learn some new strategies from the Scouts' magic-based attacks.
Shadow the Hedgehog would be an interesting addition to a crossover. In a crossover with the DC Universe, he could be compared to Batman. Shadow's brooding nature and combat skills could make him a formidable ally against villains like the Joker and Dr. Eggman. He could use his Chaos Control ability to quickly move around Gotham City or Sonic's zones, just like how Batman uses his gadgets to traverse the city.
If it's a crossover with the Power Rangers, the Rangers themselves would be obvious choices. They could combine their Zords and powers to take on King Ghidorah. Each Ranger could use their unique skills and weapons to try and damage the three - headed monster.