Well, a cool plot might be that the Ice Age animals discover a strange, glowing object which turns out to be a device from Danny Phantom's world. This device starts causing strange things to happen in the Ice Age, like creating areas of anti - gravity or making the ice melt in odd patterns. Danny Phantom shows up to retrieve the device and ends up teaching the Ice Age animals about his world of ghosts and technology while they all work together to fix the problems the device has caused.
Maybe a human hunter from Danny Phantom's world who has somehow found his way into the Ice Age. He has advanced weapons and a cruel nature. He starts hunting the Ice Age animals for sport and also wants to capture Danny Phantom because he sees Danny as some sort of strange, valuable creature. He teams up with some of the less friendly Ice Age creatures to achieve his goals.
Maybe in this crossover, some Pokemon are accidentally sent to Danny's world. Danny has to protect the Pokemon from being captured by the Guys in White while also trying to find a way to send them back. Meanwhile, a certain Pokemon, like Gengar, forms a special bond with Danny because of their shared ghostly nature.
One possible plot could be that Danny and Ben meet when a powerful interdimensional villain is causing chaos in both their universes. They initially misunderstand each other and get into a small fight. But then they realize they need to work together. Danny could use his ghost powers to phase through objects and surprise the enemy, while Ben could use his various alien forms to combat the villain's different attacks. For example, using Heatblast to counter a cold - based attack from the villain.
One interesting plot could be that a powerful ghostly villain from Danny's world somehow crosses over into the Young Justice universe. This villain starts causing chaos and Danny has to team up with the Young Justice heroes to stop it. Along the way, they learn about each other's powers and worlds.
Well, imagine if there was a device in the Danny Phantom universe that malfunctioned and created a portal to the Harry Potter world. Danny ends up at Hogwarts during the Triwizard Tournament. He gets involved in the tasks because his powers are seen as an advantage. He might team up with Harry and use his intangibility to pass through obstacles in the maze during the third task. At the same time, he could teach Harry some of his skills, like how to quickly phase through walls to avoid danger.
Well, the plot could start with a scientific experiment gone wrong in both their universes simultaneously. This creates a rift that pulls Danny into the Flash's world. There, they find out that a common enemy is trying to use the chaos to gain power. This enemy is using a device that can steal powers. Danny and the Flash team up. Danny's ability to go invisible and intangible helps them in stealth missions, while the Flash's super speed allows them to quickly reach different locations. In the end, they manage to defeat the enemy and Danny finds a way back to his own universe.
In this fanfiction, it could be that Danny Phantom meets a Danny Phantom from a parallel fan - made story. At first, they don't trust each other. But as they learn more about each other's worlds, they realize that there are some common elements that are being threatened. It could be that a powerful ghost is trying to rewrite all the fanfic universes related to Danny Phantom. So, they must combine their strengths and knowledge to stop this from happening.
Maybe there's a villain who has the ability to cross between the dimensions of Danny's world and the Young Justice world. Danny and the Young Justice team have to combine their efforts to stop this villain. Danny could teach them about dealing with supernatural threats, while they could show him how to work better in a team setting in a more traditional superhero sense.
Well, one possibility could be that the characters from Ice Age end up in the feudal Japan setting of Inuyasha. Sid could befriend Shippo, the fox demon. They might get into all sorts of comical situations together, like trying to find food in a world full of yokai. Manny, the mammoth, could be mistaken for a strange demon by some of the more superstitious villagers. And Diego might team up with Inuyasha to fight a common enemy, perhaps a powerful ice - based demon that threatens both their worlds.
A plot could involve a powerful jutsu in the Naruto universe accidentally creating a rift that leads to the Ice Age world. The Naruto characters then have to team up with the likes of Manny and Sid. For example, Sakura could use her medical ninjutsu to heal any injured animals. And in return, the Ice Age animals could show the Naruto gang how to survive in the harsh, cold environment.