For a new age eragon fanfiction, you might consider a plot where Eragon discovers a hidden civilization within Alagaësia that has been untouched by the previous wars. This civilization has its own set of rules, magic, and dragons. He could befriend some of the people there and learn from their ways. Another option is to have Eragon deal with a curse that affects all dragon riders, causing them to lose their powers gradually. He then has to go on a quest to find a cure. Additionally, a story about Eragon mentoring a young girl who has the potential to be a great dragon rider but comes from a very different background than he did would be an interesting take.
Well, a great plot for Eragon and Saphira fanfiction could be a story centered around Saphira learning a new and extremely powerful form of magic that she can't fully control at first. This causes all sorts of chaos and Eragon has to help her master it. Or perhaps a fanfic where Eragon and Saphira encounter a group of rogue dragons who have a very different view on how dragons and riders should interact. They then have to decide whether to fight them or try to persuade them to their side. Also, a story where Eragon and Saphira are magically shrunk to a very small size and have to survive in a world that is now giant compared to them could be really interesting.
One idea could be a magical exchange program. Wizards from Hogwarts and Dragon Riders from Alagaësia meet and learn from each other's magical systems. Harry Potter characters could be fascinated by the bond between Eragon and his dragon Saphira, while Eragon might be intrigued by the variety of magic at Hogwarts like Transfiguration and Potions.
One idea could be that Merlin, with his magic from the Arthurian world, meets Eragon in Alagaesia. Maybe Merlin helps Eragon to better understand and control his own magic, as Merlin has centuries of experience. He could teach Eragon about different forms of magic and how to use it more subtly.
One idea could be an OC who is a healer in Alagaësia and gets caught in the middle of the conflict between Eragon and Murtagh. They could try to use their skills to heal the wounds of both sides, physically and emotionally. Another could be an OC with a special power related to magic that neither Eragon nor Murtagh have seen before, and this power could either be a threat or an advantage in the ongoing battles. A third idea is an OC who is from a different part of Alagaësia and has a different cultural perspective on the whole dragon - rider situation. This OC could try to broker peace between Eragon and Murtagh, bringing in new ways of thinking about their roles and destinies.
One idea could be that Eragon discovers unique dragon magic only accessible to those raised by dragons. For example, he might be able to communicate with all dragons on a deeper level, not just his own. Another plot could involve him facing challenges from dragon - haters who fear his special bond with the dragons. They might try to break the bond or harm him.
Well, perhaps Eragon's dragon, Saphira, is injured and they have to seek out the healing magic in Skyrim. Along the way, they meet the Dragonborn. The two dragon - related characters could either become friends or rivals. Eragon might be amazed at the variety of dragons in Skyrim, some of which are different from the ones he knows. And he could share his knowledge of dragon - riding with the Dragonborn.
One plot idea could be that Ichigo and Eragon meet in a dimension that is a cross - between their two worlds. They have to team up to fight a common enemy that is threatening both of their universes. Their different powers, Ichigo's Soul Reaper abilities and Eragon's dragon - riding and magic, would make for an interesting combination in battles.
One idea could be that Eragon and Saphira travel back to a time when the Dragon Riders were first formed. They could witness the original pacts between dragons and riders, and perhaps even influence the early development of their order in a positive way, like sharing some of the knowledge they have from their own time.
One interesting plot could be the discovery of a new type of magic that was long forgotten in the wizarding world. Maybe it has the power to heal the most incurable dark magic curses.
One idea could be to explore the backstory of Scrat and his never - ending quest for the acorn in more detail. Maybe show his early days and how he first discovered the acorn. Another idea is to create a fanfiction where the animals have to deal with a new type of natural disaster in the Ice Age world, something different from what we've seen in the movies. A third idea could be to write about a friendship that develops between two unlikely animals during the Ice Age.