The Keyblade wielders like Sora, Riku, and Kairi would bring a sense of adventure and a different kind of magic to Equestria. The ponies, on the other hand, could teach them about the power of friendship in a more direct way. Donald and Goofy might be a bit startled by the ponies at first but would soon get used to them. Donald's magic could be compared to the magic of some of the unicorn ponies, and they could exchange magical knowledge.
Riku could also play a major role. His more complex and sometimes brooding personality could add an interesting dynamic. He might start off as a bit of an outsider in Equestria but eventually come to understand and care for the ponies, especially if there are some ponies who remind him of his friends from his own world.
Sora would likely be very protective of Ariel. He would see her as someone in need of help, just like he does with many characters in different worlds. Ariel, on the other hand, would be fascinated by Sora's world and his Keyblade. She might ask him a lot of questions. Donald could get frustrated with Ariel's overly curious nature at times, while Goofy would just be friendly and go along with whatever is happening.
One interesting plot could be that the ponies from MLP accidentally find themselves in the Kingdom Hearts universe. They might mistake the Heartless for some kind of dark magic creatures from their own world and try to fight them using their magic. For example, Twilight Sparkle could use her powerful spells to combat the Heartless while trying to figure out how to get back to Equestria.
One idea could be that Sora and his friends end up in Equestria. They could team up with the ponies to fight a new threat that is affecting both their worlds. Maybe the Heartless are somehow being drawn to Equestria's magic and they need to find a way to stop it together.
The Pokemon could befriend the ponies. For instance, a friendly Bulbasaur might help the ponies in their gardens by using its grass - type abilities. The ponies, in return, could offer the Pokemon shelter and food. They could also learn from each other. Ponies could teach Pokemon about their culture, like the different pony tribes and their traditions, and Pokemon could show ponies their unique battle strategies.
The idea of friendship is also quite common. Since MLP is big on friendship, fanfictions often use that concept within the context of Kingdom Hearts' more battle - and adventure - oriented world. For example, the ponies' friendship might be a source of power against the darkness in Kingdom Hearts. And also, the concept of different worlds is shared. Just as Kingdom Hearts has various Disney - inspired worlds, MLP's Equestria can be seen as another world in the Kingdom Hearts universe in these fanfictions.
Scooby would probably be really curious about the ponies. He might sniff around them and try to figure out if they were some kind of strange dog - like creatures. Shaggy would be friendly right away, especially if the ponies offered him some food. The ponies, like Pinkie Pie, would be very excited to meet these new and strange visitors. She might throw a big party for them.
The ponies would probably be terrified at first. The Doomguy, on the other hand, would be confused by these small, colorful creatures. But eventually, they might find common ground. For instance, they could both be fighting for survival in a strange world. The ponies could offer their magic to help the Doomguy, and he could offer his combat skills to protect the ponies.
The characters could interact through a portal that connects their two worlds. For instance, Applejack could enter the Undertale world and meet Toriel. Applejack's honesty and hard - working nature might impress Toriel, and they could exchange stories about taking care of others (Applejack on the farm and Toriel with the lost children).
The characters could interact through shared quests. For instance, if there's a dark force emerging in both worlds, they might work together to defeat it. Link could teach the ponies some combat skills like sword - fighting (maybe in a pony - friendly way), and the ponies could teach Link about their unique magic and how it works in Equestria.