The main antagonist could be a powerful Ice Age beast that has been corrupted by dark chakra. This beast might see Naruto and his friends as invaders in its territory and attacks them without mercy. It could have abilities like freezing its opponents with a single breath or being able to move at incredible speeds on the icy terrain.
One interesting plot could be that Naruto and his friends are transported to the Ice Age world. There, they have to deal with the harsh environment and new, prehistoric creatures. Maybe Naruto tries to use his ninja skills to hunt for food or build shelters, but quickly realizes that his usual techniques need to be adapted to this new world. For example, his Rasengan might not be as effective against a woolly mammoth as it is against a human enemy.
It could be really cool. Maybe Naruto and his friends could end up in the Ice Age world. They might use their ninja skills to help the Ice Age characters. For example, Naruto could use his shadow clone jutsu to gather food for the prehistoric animals more quickly.
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.
Well, it could be that Naruto somehow gets transported to the Ice Age 2 world. Maybe he uses his ninja skills to help the animals there. For example, he could use his shadow clone jutsu to gather food more quickly for the starving animals during the ice age.
The appeal of Ice Age Shira fanfiction lies in the character herself. Shira is a complex character. She has a tough exterior from her pirate days but also a softer side. Fanfiction allows readers and writers to explore that duality more deeply.
Manny from the Ice Age could be very interesting. In a crossover, he would be the big, strong and somewhat skeptical character towards the Naruto bunch. He has his own code of honor in protecting his herd, and seeing how he interacts with the ninja, especially with their strange powers like Naruto's Rasengan, would be really cool. He might start off not trusting them but eventually come to respect them.
Definitely Sakura and Sid. Sakura is a strong - willed and intelligent character. Sid is a bit of a goofball. Sakura could be constantly exasperated by Sid's antics, but at the same time, Sid could bring out a more fun - loving side of Sakura. And Sakura could use her medical ninjutsu to help Sid if he gets injured during their adventures in the Ice Age world.
I think Naruto and Manny would be an interesting combination. Manny is this big, strong mammoth, and Naruto is a skilled ninja. Their personalities are so different. Naruto is full of energy and always eager to prove himself, while Manny is more of a calm and protective leader. It would be fun to see how they interact and how Naruto's ninja skills might come in handy in the Ice Age world with Manny's brute strength.
Some Pokemon might grow thicker fur or develop ice - like coverings. For instance, Eevee could evolve into a new form with white fur and ice crystals on its tail. Fire - type Pokemon might have a frosty sheen on their bodies as a sign of the cold affecting them. Water - type Pokemon could have icicles forming on their fins.